Question 363264
Divide 18 by 7 which is 2.57
This tells you that the tank can consume 7 gallons of gas 2.57 times before it runs out of gas. 
Multiply the number of acres each 7 gallons will cut, which is 4 acres by the 2.57 This will give you 10.28
You can cut 10.28 acres with 18 gallons of gas.