Question

Joe is 5 years older than sally. in 15 years joe will be 3 times as old as sally is now. how old is sally now

Answer 1

Lets set up an equation for this:
Let J = Joe
S = Sally

J = 5 + S
15 + J = 3S

Now solve for S (Sally) by substituting J (Joe) in the 2nd equation:
15 + (5 + S) = 3S
20 = 2S
S = 10

Therefore Sally is 10 years old.