One number is 5 greater than another number. their sum is equal to 5 times their difference. Find the numbers.

1)x=y+5
2)x+y=5(x-y)

1) tells us x-y=5
therefore we can repalce x-y with 5 in 2)

new system:

x+y=25
x-y=5 

adding the two equations...

2x=30

x=15
so...

y=10