Three quarters of the boats in the marina are white. 4/7 of the remaining boats are blue, and the rest are red. If there are 9 red boats, how many boats are in the marina?

Total number of red boats in Marina = 9

Let us assume the total number of boats in Marina = x

Then

Total number of white boats = 3x/4

Remaining number of boats = x - (3x/4)

= (4x - 3x) / 4

= x/4

Number of blue boats in Marina = (4/7) * (x/4)

= x/7

Number of red boats in Marina = (x/4) - (x/7)

= (7x - 4x) / 28

= 3x/28

Then we can write the equation as

3x/28 = 9

3x = 28 * 9

3x = 252

x = 252/3

= 84

So the total number of boats in Marina was 84.