A shopkeeper reduces the price of a pen by 25% as a result of which the sales quantity increased by 20%. If the revenue made by the shopkeeper decreases by x% then x is

Revenue decreases by $10\%$. Multiplying the price factor $0.75$ by the quantity factor $1.20$ gives $0.90$ of the initial revenue. The value $25\%$ accounts for the price reduction alone and leaves out the increase in units sold.