Question

Antonio has a total of 55 marbles. He has 15 green marbles and 12 red marbles. The rest of the marbles are blue. How many blue marbles does he have?

A student solves this problem by writing an equation. Which strategy could help check the student’s answer?

Answer 1

total = green marbles + red marbles + blue marbles
55 = 15 + 12 + blue marbles
55 = 27 + blue marbles
55 - 27 = blue marbles
28 = blue marbles

Answer 2

Antonio has
`28` blue marbles

Explanation:

Let

x------> the number of green marbles

y------> the number of red marbles

z------> the number of blue marbles

we know that

`x+y+z=55` -----> equation A

`x=15` ----> equation B

`y=12` ----> equation C

Substitute equation B and equation C in equation A and solve for z

`15+12+z=55`

`27+z=55`

Subtract
`27` both sides

`27+z-27=55-27`

`z=28`