1oz of walnuts were mixed with 4 oz of peanuts which cost $4/oz to make mixed nuts which cost $5/oz. what is the price per oz of walnuts

Call w the unknwon price of the walnuts, then:

total cost of walnuts = 1oz *  w

Total cost of peanuts = 4 oz * $4/oz = $16

Total cost of nuts = 5oz * $5 /oz = $25

=> 1oz*w + $16 = $25 => w = ($25 - $16) /1oz = $9/oz

Verfify:

1oz * $9/oz + 4oz * $4/oz = $9 + $16 = $25, which is equal to 5oz * $5/oz.

$9 per oz.

7.666270253x10 raised to the 18th power