Question

Sarah's age is five less than twice her sister's age. Sarah is fifteen years old.

Which equation could you use to solve for the age of Sarah's sister?

2x - 5 = 15
2(15) - 5 = x
15 = 5 - 2x
15 - 5 = 2x

Answer 1

The first one would be correct 

Answer 2

let her sisters age be x.
=>sister .....x

Sarah.
twice x=2x. then less than 5
=> (2x-5 )
if Sarah = 15 years
by substation, 2x-5=15