Alan is 5 years older than bob. bob is twice as old as cindy. all their ages add up to 20. what are their ages?

We can use a variable equation to solve this. Let A represent alan's age, B represent bob's age, and C represent Cindy's age. So Bob's age would be Alan's age - 5, or a -5 = B. Then Bob's age would be double Cindy's age, so that means 2 multiplied by Cindy's age = Bob's age, or 2C = B. Your two equations are: 

A - 5 = B
2C = B

Also a third equation that helps is A + B + C = 20.

Finally, here are the ages. 
Alan = 11 years old
Bob = 6 years old
Cindy = 3 years old.

PS: Sorry, I'm not that good at explaining things, but here are the answers