Here the price of 1 gallon of milk is $3.79. SO price of 3 gallons of milk = 3.79 * 3= $11.37. But she gives the cashier $20 .
So to find the change Kari shoul receive, we need to subtract the price of 3 gallons of milk from $20 = 20 -11.37 = $8.63 .
THerefore Kari receive $8.63 as change from cashier .