Respuesta :

Let x be the litres of 10% solution

0.1x + 0.5 (40) = 0.4 (40 + x)

x + 5 (40) = 4 (40 + x)

x + 200 =  160 + 4x

3x = 40

x = 40/3L or 13 1/3L.

Therefore, 40/3L or 13 1/3L of 10% solution.

Good luck :)