John is twice as old as his son. In five years John's age will be three times what his son's age was six years ago. How old are they?
Let J stands for John and S for his son.
J=2S. In five years: J+5=3(S-6). Substitute 2S for J in second equation and we get: 2S+5=3S-18. When we solve it we get S=23. Substitute 23 in J=2S and we get J=46.