To solve this, you can make a proportion with gallons of milk as the numerator and the cost as the denominator: 4/16.76=7/x, where x=the cost for 7 gallons of milk. Then, you cross multiply to get 4x=7*16.76 or 4x=117.32. Then, you divide both sides by 4 to get x=$29.33.
I hope this helps.