The discount for each gallon of whole milk is $3 - $2 = $<<3-2=1>>1.
So the discount for 3 gallons of whole milk is $1/gallon x 3 gallons = $<<1*3=3>>3.
The total discount for 5 boxes of cereal is $1/box x 5 boxes = $<<1*5=5>>5.
You will save $3 + $5 = $<<3+5=8>>8 for 3 gallons of whole milk and 5 boxes of cereal.
#### 8