If Doug, Betty's friend, is 40 years old, and the sum of their ages is 90, Betty is 90-40 = <<90-40=50>>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 = $<<2*50=100>>100
If Betty wants to buy 20 packs of the nuts, she'll have to pay 20*$100 = $<<20*100=2000>>2000
#### 2000