5 liters (L) of 30% bleach solution contains 0.3×(5 L) = 1.5 L of bleach.
3 L of 50% bleach contains 0.5×(3 L) = 1.5 L of bleach, too.
Combined, you would have 8 L of solution containing 1.5 L + 1.5 L = 3 L of bleach, so the concentration of bleach is
(3 L) / (8 L) = 0.375 = 37.5%
