let

x = the liters of water that must be added

note that the water is 0% acid solution

0% of acid is x l water + 20% acid in 7 l of water = 10% acid in (x + 7) l solution

0*x + 0.2*7 = 0.1*(x + 7)

0.1*x + 0.7 = 1.4

x = (1.4 - 0.7)/0.1

x = 7 l

we must add 7 liters of water.