Question

In order to solve the following system of equations by subtraction, which of

the following could you do before subtracting the equations so that one
variable will be eliminated when you subtract them?
4x – 2y=7
3x – 3y=15

A. Multiply the top equation by-3 and the bottom equation by 2.

B. Multiply the top equation by 1/3.
C. Multiply the top equation by 3 and the bottom equation by 4.

D. Multiply the top equation by 15 and the bottom equation by 7.

Answer 1

Answer: I think it's C.Multiply the top equation by 3 and the bottom equation by 4

Explanation:

Multiplying the top equation by 3 you get:

3(4x – 2y) = 3∗7

12x - 6y = 21 (eq.1)

Multiplying the bottom equation by 4 you get:

4(3x – 3y) = 4∗15

12x - 12y = 60 (eq.2)

Now, if you subtact eq.2 from eq.1 you get:

(12x - 6y = 21 )

-

(12x - 12y = 60)

___________________

0x + [- 6y -(-12y)] = 21 - 60

Carrying out the calculation you get:

6y = -39

y = -39/6

y = -13/2