Question 17680
the sum of two numbers is 42.  One number plus two times the other number is 57.  What are the numbers?  The answers are 15 and 27 but I don't understand 

let one number be =x
let the other number be =y
their sum =x+y=42..........(1)
two times the other number =2y
plus the first number x = x+2y=57...........(2)
(2)-(1) ....x+2y-x-y=57-42=15
y=15...substituting in (1),we get ..x+15=42
x=42-15=27