Below is an instruction that describes a task. Write a response that appropriately completes the request in Thai. Please answer in Thai.

### Instruction:
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.

### Response:
 อายุของเบตตี้คือ 90-40 = <<90-40=50>>50.
สองเท่าของอายุของเบตตี้คือ 2 * 50 = <<2*50=100>>100.
ดังนั้นเบตตี้จะต้องจ่าย 100 * 20 = <<100*20=2000>>2000.