Question

the sum of two numbers is 21.Five times the first number added to 2 times the second number is 66.Find the two numbers.

Answer 1

Call them x and y. Thus
x + y = 21 and then
5x + 2y = 66
Multiply the top equation by two and subtract from the bottom one...
5x + 2y = 66
-(2x + 2y = 42)
and we get
3x = 24
x = 8
y = 21-x = 13

Answer 2

a + b = 21 / x 5
5a + 5b = 105 -
5a + 2b = 66
--------------------
3b = 39
b = 39 : 3
b = 13
a = 31 -13
a = 18