Pour out 2/5 of the solution and fill it with water, and the concentration becomes (3/5) X.

After pouring out another 3/4, the remaining amount is (1/4) (3/5) x.

85% salt water is filled, and the addition amount is (3/4)85%.

So (1/4) (3/5) x+(3/4) 85% = x.

X=75%