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

### 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:
 Umri mara mbili wa Betty ni gharama ya pakiti ya karanga. Ikiwa B inawakilisha umri wa Betty na gharama ya pakiti ya karanga ni 2B, basi jumla ya gharama ni 20 * 2B = <<20*2B=40B>>40B.
Jumla ya umri wao ni 90, hivyo B + 40 = 90. Kwa kumaliza B, tunapata B = 50.
Hivyo jumla ya gharama ni 40 * 50 = <<40*50=2000>>2000.