Let J represent the number of tickets Jose sells. Let M represent the number of tickets Mark sells. Make equations, if Mark sells 15 more tickets than Jose, the equation is J+15=M. Because we know that in between both of them they sell 63 raffle tickets, then M+J=63. Now plug in the value of M (J+15) into M+J=63. And you get (J+15)+J=63, which is 2J+15=63. Subtract 15 from both sides and get 2J=48. Now divide 48 by 2, J=24. So now you know Jose sold 24 tickets, and to find how many Mark sold, plug in 24 into J+15=M and get 24+15=M where M is 39. Now check, 24+39=63. So Mark sold 39 tickets and Jose sold 24.