Answer: the number of boats that are in the marina is 105.
Step-by-step explanation:
Let x represent the number of boats that are in the marina.
Two-thirds of the boats in the marina are white. It means that the number of white boats in the marina is
2/3 × x = 2x/3
The remaining number of boats will be
x - 2x/3 = x/3
4/7 of the remaining boats are blue. It means that the number of boats that are blue is
4/7 × x/3 = 4x/21
The number of boats left would be
x/3 - 4x/21 = 3x/21
If there are 15 red boats, it means that
3x/21 = 15
3x = 21 × 15 = 315
x = 315/3
x = 105
