Let x be the amount of salt in the solution.
At day's end, the concentration of salt in the beaker is
x / (4 L) = 0.08 = 8%
Solve for x to get
x = 0.08 (4 L) = 0.32 L
So at the beginning, the concentration of salt in the beaker was
(0.32 L) / (5 L) = 0.064 = 6.4%
