Bill bought 10 lbs of peanuts and cashews for his party. The cashews cost $ 7/ lb and the peanuts $3/ lbs. Bill spent a total of $50.

let P be the weight of peanuts he bought. So the weight of Cashews is 10 - P.

3P + 7(10 - P) = 50

3P + 70 - 7P = 50

3P - 7P = 50 -70

-4P = -20

P = 5

So, Bill bought 5 pounds of peanuts and 5 pounds of cashew.