Question

a+b=5, 3a+2b=20, determine the value of 3a+b?

(1) 20

(2) 15

(3) 10

(4) 25

 

Answer 1

First equation a+b = 5

Multiple to each side by negitive 3.

-3a-3b = -15 ---> (1)

Second equation 3a+2b = 20 ---> (2)

To eliminate the a value add the equations (1)&(2).

-3a-3b = -15

3a+2b = 20

___________

-b = 5

Multiple to each side by negitive one.

b = -5.

Substitute the b value in (2).

3a-10 = 20

Add 10 to each side.

3a-10+10 = 20+10

3a = 30

Divide to each side by 3.

3a/3 = 30/3

a = 10

Now 3a+b = 30-5 = 25

Option 4 is correct choice.