Question 232912
lots of possible ways to find this answer I honestly am not sure which is best I just use trail and error. first I think of all the possible statements

Peter x 2 = john 
John/2 = peter
peter -5 = alice 
alice + 5 = peter
alice x 3 = john
john/3 = alice

now that I have that in my head, I ask my self what age would john be when alice was just born. if you force yourself to do a problem like this in your head it will help you in the future. think about the possibilities  will if john is 10 divide that by 2 which mean peters is 5 subtract 5 and alice was just born she is 0.  now in 5 years alice will be 5 and john 15 divided john 3 it that proves alice is 5. add 5 to alice and we get john who is now 10 which is the answer. 

**note it may be easier for you to start with the youngest of the age group and work from there.