I might be wrong, but I think you could solve this using algebra (if you haven't studied algebra you probably won't understand this answer). First I would try to let Henry's age equal X (it's the unknown) and let his sister's age equal his age (X) minus three, since she's three years younger. So:
Henry's age = X
Sister's age = X - 3
Total = 180
If you knew there two ages adding them together would get 180. That's what they mean by the "product" of their ages. Since we're letting the variables represent there ages we'll add those instead.
X + X - 3 = 180. Next add the X's
2X - 3 = 180. Add 3 to both sides
2X = 183. Divide 2 into 183
X = 91.5
X - 3 = 88.5
So Henry is roughly 91 and his sister is roughly 88. They only asked for Henry's age so depending on the context you can give them his exact age or his rough age. I think I did this right, so hopefully this helps!
