Hey,
If you mean brother's age is 1/3 of her age, than he is 6. When 18 year passes and the girl becomes 36, he will become 6 + 18 = 24.
If you mean brother's age is threefold of her age, than now it's 54. When 18 year passes and she becomes 36, he will become 54 + 18 = 72
Hope this helps and don't forget to choose brainliest please :)
