Sensim Math Original · sm-4
Easy mode Grade 4Problem
Imagine a bank teller writing down the day's money on a ledger. For fun, the teller uses only the digit 3 in every amount.
By the end of the day the teller has four amounts coming in and one amount going out. Add the four amounts together, then subtract the last one:
33,333+3,333+333+33−3.What is the units digit (the ones-place digit) of this final number?
Pick an answer.
Try it yourself first — the explanation is most useful after you’ve attempted it.
Toolkit + CCSS Solution
Understand
Restated: The bank teller records four credits ($33{,}333$, $3{,}333$, $333$, $33$) and one debit ($3$). We need only the units (ones) digit of the net balance $33{,}333 + 3{,}333 + 333 + 33 - 3$, not the full dollar amount.
Givens: Four credit entries: $33{,}333$, $3{,}333$, $333$, $33$; One debit entry: $3$; Every entry is built entirely from the digit 3; Every entry ends in the digit 3 (units digit is 3 for every term); Mixed sign pattern: four additions and one subtraction; Five answer choices: (A) 0, (B) 3, (C) 5, (D) 7, (E) 9
Unknowns: The units digit of $33{,}333 + 3{,}333 + 333 + 33 - 3$
Understand
Restated: The bank teller records four credits ($33{,}333$, $3{,}333$, $333$, $33$) and one debit ($3$). We need only the units (ones) digit of the net balance $33{,}333 + 3{,}333 + 333 + 33 - 3$, not the full dollar amount.
Givens: Four credit entries: $33{,}333$, $3{,}333$, $333$, $33$; One debit entry: $3$; Every entry is built entirely from the digit 3; Every entry ends in the digit 3 (units digit is 3 for every term); Mixed sign pattern: four additions and one subtraction; Five answer choices: (A) 0, (B) 3, (C) 5, (D) 7, (E) 9
Plan
Primary tool: #9 Solve an Easier Related Problem
Secondary: #5 Look for a Pattern, #3 Eliminate Possibilities
A five-digit credit plus three more credits and one debit looks like a heavy column-subtraction problem, but the only column that affects our answer is the ones column. Replacing each multi-digit entry with just its units digit collapses the whole ledger to single-digit arithmetic (Tool #9). The shared 'every entry ends in 3' fingerprint is the pattern (Tool #5) that makes the reduction clean, and Tool #3 lets us cross-check against the choices at the end.
Execute — Answer: E
4.NBT.A.2 Step 1 - Every ledger entry — $33{,}333$, $3{,}333$, $333$, $33$, and $3$ — ends in the digit 3.
- The units digit of a sum or difference depends only on the ones column of each term, so we may ignore the tens, hundreds, thousands, and ten-thousands columns entirely.
- This is the easier-related-problem move: five multi-digit numbers shrink to five copies of the digit 3.
💡 Reading the rightmost digit of a multi-digit number is the Grade 4 place-value skill.
2.OA.B.2 Step 2 - Add the four credit units digits and then subtract the one debit units digit, just as the original signs prescribe: $3 + 3 + 3 + 3 - 3$.
- Compute left to right: $3 + 3 = 6$, $6 + 3 = 9$, $9 + 3 = 12$, $12 - 3 = 9$.
- So the ones column of the net balance is 9.
💡 Adding and subtracting four small numbers under 20 is Grade 2 mental arithmetic — the repeated 3s make the pattern obvious.
4.NBT.B.4 Step 3 - Confirm that no carry or borrow from the tens column muddies the ones place.
- In the actual sum the four credits' ones columns add to $12$, which puts a 2 in the ones place and carries a 1 — but then the debit subtracts a 3 from a tens column that has plenty to spare, so no borrow reaches back into the ones column.
- Either way the ones digit of the final balance is the value we just computed, namely 9.
💡 Grade 4 multi-digit add-subtract reasoning lets us check that no carry/borrow leaks across columns once the signs are settled.
2.NBT.A.1 Step 4 - Match against the five offered digits.
- The candidates are 0, 3, 5, 7, 9.
- Our ones digit is 9, which is exactly choice (E).
- The other digits — 0, 3, 5, 7 — never arise from $3+3+3+3-3$ in the ones column, so they are ruled out.
💡 Reading off the ones place to pick a multiple-choice option is Grade 2 place-value work.
4.NBT.A.2 Every ledger entry — $33{,}333$, $3{,}333$, $333$, $33$, and $3$ — ends in the d 2.OA.B.2 Add the four credit units digits and then subtract the one debit units digit, ju 4.NBT.B.4 Confirm that no carry or borrow from the tens column muddies the ones place. In 2.NBT.A.1 Match against the five offered digits. The candidates are 0, 3, 5, 7, 9. Our one Review
Reasonableness: Sanity check by computing the full balance: $33{,}333 + 3{,}333 = 36{,}666$; $36{,}666 + 333 = 36{,}999$; $36{,}999 + 33 = 37{,}032$; $37{,}032 - 3 = 37{,}029$. The ones digit of $37{,}029$ is indeed 9, so the units-digit shortcut agrees with the full ledger arithmetic and the answer (E) 9 is correct.
Alternative: An equivalent route is Tool #13 (Convert to Algebra) using modular arithmetic mod 10. Each entry is $\equiv 3 \pmod{10}$, so the net balance is $3 + 3 + 3 + 3 - 3 \equiv 9 \pmod{10}$. Same digit, but for a young solver the easier-problem move reads more naturally than naming the modulus.
CCSS standards used (min grade 4)
2.NBT.A.1Understand that the three digits of a three-digit number represent hundreds, tens, and ones (Reading the ones place of the net balance to match it against the offered answer choices.)2.OA.B.2Fluently add and subtract within 20 using mental strategies (Combining the five units-digit terms $3 + 3 + 3 + 3 - 3 = 9$ in one's head.)4.NBT.A.2Read and write multi-digit whole numbers and compare using symbols (Recognizing that each ledger entry — up to $33{,}333$ — has units digit 3.)4.NBT.B.4Fluently add and subtract multi-digit whole numbers (Checking that no carry or borrow from the tens column reaches into the ones column once the four credits and one debit are combined.)
⭐ This AMC 8 problem only needs Grade 4 multi-digit place-value sense you already know!
⭐ This AMC 8 problem only needs Grade 4 multi-digit place-value sense you already know!
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