Question 82355
Let number of pounds of Raisins = R
Number of pounds of Pineapple  = P
Total weight of mixture = 20
P+R = 20

price per pound of raisins = 3
Total price of raisins = 3 X R = 3R
Price per pound of pineapple = 4
Total price of Pineapple = 4 X P = 4P

We are making 20pounds of mixture worth 3.60 per pound
SO total price of the mixture = 20 x 3.6 = 72

So 3R + 4P = 72
   R  +  P = 20

Can you solve from here?