Dave is three times as old as his sister. In seven years the sum of their ages will be 38. How old are they now?

Dave is 18 and his sister is 6.

Step-by-step explanation:

If you do 38 - 7x2 you can work out the combined ages of them now which is 24 (you have to do 7x2 because there are two of them and its seven years in the future). Then you can work out their ages using a ratio. Dave's age : his sister's age is 3:1 so if you divide 24 by 4 (3+1 = 4 so your can work out one portion of their age) you can find out how old the sister is which is 6. Times that by 3 to get Dave's age which is 18. A formula for this (where x is his sisters age) would be 3x=38-7x2 which cancels down to 6.