If 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?

Sally is 10. To solve: Let's use as variables the letter j for Joe's age today and s for Sally's age today, and try to solve for s. Start by expressing what we know about the relationships of Joe's and Sally's ages as equations. Joe (today) is 5 years older than Sally (today): j = s + 5 Joe in 15 years will be 3 times Sally's age (today): j + 15 = 3s Begin solving the two equations simultaneously by replacing the j in the second equation with its value from the first; then solve for s: j + 15 = 3s s + 5 +15 = 3s [since j = s + 5] s + 20 = 3s 3s = s + 20 [just reversing the equality] 2s = 20 [subtracting s from both sides] s = 10 [Your answer! Sally's age today.] Now solve for Joe's age today (j), and check the answer: j = s + 5 [given] j = 10 + 5 [substituting the solved age for Sally] j = 15 [Joe's age today!] Checking we see that Joe's age is 5 more than Sally's; and we see that in 15 years, Joe's age of 30 is 3 times Sally's age today of 10. Success!