Let 'x' be the amount of 60% banana solution and
Let 'y' be the amount of 40% banana solution to make 50% banana solution
given , x+y =100
also,
50% of(x+y) = 60% of x + 40 % of y
5x + 5y = 6x + 4y
x =y
putting this in x+y =100
so, x = y = 50
so, amount of each solution is needed to make a 100L solution that is 50% banana juice is 50L