Dash+dash=100
Both dash and dashare prime numbers.
How many different solutions can you find​

6

Explanation:

97+3 = 100

89+11 = 100

83+17 = 100

71+29 = 100

59+41 = 100

53+47 = 100

I counted them using excel, see pic.

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.