Set up an expression and compute in order

3.OA.A.33.OA.D.8 · take · grade 3

Archetype: Apply a Newly Defined Operation · step in a 5-type progression

One eraser costs $\$0.90$ and one pair of scissors costs $\$0.95$ . The teacher wants to give every one of the $30$ students in Mia's class one eraser and one pair of scissors. How much money is needed in all to buy the erasers and the scissors?

Show solution

Understand

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.

Givens

One eraser costs $0.90.
One pair of scissors costs $0.95.
There are 30 students, each getting one of each item.

Unknowns

The total money needed to buy 30 erasers and 30 pairs of scissors.

Constraints

Every student receives exactly one eraser and one pair of scissors.

Plan

#7 Identify Subproblems · also uses: #8 Analyze the Units

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.

Execute

#7 Identify Subproblems 3.NBT.A.2

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.

#8 Analyze the Units 3.OA.A.3

Each of the 30 students costs

1.85, so multiply

1.85 by 30. Think of it as 1.85 times 3 tens:

1.85 \times 3 = \$5.55

, then times 10.

\$1.85 \times 30 = \$55.50

Equal groups of money means repeated addition, which is multiplication.

Answer: $55.50

Review

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.

Compute the two totals separately: 30 erasers cost $0.90 \times 30 = \$27.00 and 30 pairs of scissors cost$ 0.95 \times 30 = $28.50; their sum is $55.50.

Standards · min grade 3

3.OA.A.3 Solve multiplication and division word problems within 100 — Multiplying the per-student cost by the number of students.
3.NBT.A.2 Fluently add and subtract within 1000 — Adding the eraser and scissors prices for one student.

💡 Find what one kid costs, then multiply by how many kids -- that is all Grade 3 thinking!