A first number plus twice a second number is 5. Twice the first number plus the second totals 13. Find the numbers.

Let a and b = the numbers we must find.

a + 2b = 5Equation A

2a + b = 13Equation B

We have a system of equations in two variables.

I will solve A for little a.

a + 2b = 5

a = 5 - 2b

Now I will plug 5 - 2b for a in B.

2(5 - 2b) + b = 13

10 - 4b + b = 13

-3b = 3

b = -1

To find a, replace b with -1 in EITHER A or B.

I will use Equation A.

a + 2b = 5

a + 2(-1) = 5

a - 2 = 5

a = 7

The two numbers are -1 and 7.

Understand?

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