A single discount which is equivalent to a series of discounts p%, (p/2)%, (p/4)% is 31.6%. What is the value of p?

Effective remaining fraction after series of discounts = (1 - p/100)(1 - p/200)(1 - p/400) = 1 - 0.316 = 0.684. Try p = 20: (1 - 0.20)(1 - 0.10)(1 - 0.05) = 0.80 × 0.90 × 0.95 = 0.684. This matches exactly. So p = 20%.