now you have
(.15)0.5 lb peanuts = 0.075 lb peanuts
0.5 lb nuts 0.5 lb nuts
for every pound of 75% mixed nuts you add, you get .75 lb peanuts, how many pounds, x, do you need to add to get the ratio 50 to 100?
0.075 lb peanuts + 0.75x lb peanuts = 50
0.5 lb nuts + x lb nuts 100
cross multiply
(0.075 + .75x) 100 = (0.5 + x) 50
7.5 + 75x = 25 + 50x
25x = 17.5
x = 0.7 lb of 75% peanuts