Twice Betty's age is the cost of a pack of nuts. Doug, Betty's friend, is 40 years old. If the sum of their ages is 90, and Betty wants to buy 20 packs of the nuts, calculate the amount of money she'll pay for the packs of nuts. Think carefully first, then make a decision:
If Doug, Betty's friend, is 40 years old, and the sum of their ages is 90, Betty is 90 - 40 = 50 years old. Since twice Betty's age is the cost of a pack of nuts, a pack of nuts is sold at 2 * $50 = $100. If Betty wants to buy 20 packs of the nuts, she'll have to pay 20 * $100 = $2000. So, the answer is 2000.