Problem
Set up an expression and compute in order
Each of 30 students gets one eraser (0.90) and one pair of scissors (0.95). Find the total cost for all the erasers and all the scissors together.
Operations
Your answer
dollars
Watch out Don't stop at the cost for one student ($1.85) — multiply that by all 30 students to get the total.
How to solve
Strategy Identify Subproblems — Each student's cost is one small subproblem (eraser + scissors). Find the cost for one student, then multiply by 30. Tracking dollars keeps the arithmetic honest.
1STEP 1
Cost for one student
Add the price of one eraser and one pair of scissors to get the cost of supplies for a single student.
$0.90 + $0.95 = $1.85
Combining two prices is just adding money amounts, a Grade 3 addition skill.
3.NBT.A.2Identify Subproblems2STEP 2
Cost for all 30 students
Each of the 30 students costs $1.85, so multiply $1.85 × 30 to get $55.50 in all.
$1.85 × 30 = $55.50
Equal groups of money means repeated addition, which is multiplication.
3.OA.A.3Analyze The UnitsThe total money for all 30 students equals one student's cost of $1.85 multiplied by 30.
Why?
All 30 students cost the same 1.85, and combining equal groups is what multiplying by 30 does.
Why?
Each student receives one eraser and one pair of scissors at the same two prices, so every student's cost comes out to the identical $1.85.
Why?
One student's cost is the eraser part 0.95, and those two parts add back to the whole $1.85.
🧱Whole is the sum of its partsBreak a thing into pieces with no gaps or overlaps and the pieces add back to the whole.
Why?
Adding 1.85 times 30 means.
🧱Multiplication as equal groupsa times b means a equal groups of b — it is just repeated adding of the same amount.
Answer
55.50 dollars
$1.85 × 30 = $55.50
Each student costs about 2, and 2 times 30 is about 60. Our exact answer 55.50 is just under that estimate, so it is reasonable.
Takeaway
Find what one kid costs, then multiply by how many kids -- that is all Grade 3 thinking!
- Cost for one student
- Cost for all 30 students
Where next?
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