Answer:
Step-by-step explanation:
Let x be the number of male fish. Then 2x-15 is the number of female fish, and the ratio of female to male fish is ...
 0.65/0.35 = (2x -15)/x . . . . . rational equation
 13/7 = 2 - 15/x . . . . . . . . . . . . simplified
 15/x = 2 -13/7 = 1/7 . . . . . . . . add 15/x - 13/7
 x = 7·15 = 105 . . . . . . . . . . . . multiply by 7x
 2x-15 = 210 -15 = 195 . . . . . . use the expression for the number of females
There are 195 female fish and 105 male fish in the tank.