The key to solve this kind of questions is:
1. add all sides of your ratio (3 + 5 = 8)
2. Divide the total by the sum you obtained in 1. So, (72 / 8 = 9)
3. Now times each side of your ratio with the answer you obtained in 2. So,
3 * 9 = 27
and 5 * 9 = 45
Those are your two groups. To double check you can add 27 + 45 = 72.
Hope that helped!