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

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.