Question 213074
Let  x =  water  added
.
x (0%) + 100ml * .9 = (100+x) * .4,,,,,salt  balance
.
 90 = 40 + .4x
.
-.4x = -50
.
x= 125 ml,,,,h2o  added
.
Total  volume  is  100 + 125  = 225  ml
.
check,,,,100*.9 = 225* .4 = 90,,,,,ok