Answer:50 customers would be expected to buy birthday cards next month.
Explanation:
Last month customers at a gift shop bought 40 birthday cards, 19 congratulation cards, 20 holiday cards, 21 thank you cards. The total number of greeting cards that the customers bought last month would be
40 + 19 + 20 + 21 = 100
Percentage of customers that bought birthday cards last month would be
40/100 × 100 = 40%
supposed 125 customers by greeting cards next month, the expected number of customers that would buy birthday cards next month would be
40/100 × 125 = 50