Answer:
The container can hold 18 gallons
Step-by-step explanation:
Assume the container can hold x gallons
∵ The container is  full with water
  full with water 
∵ It can hold x gallons
→ Multiply  by x to find the number of gallons in the container
 by x to find the number of gallons in the container
∴ The number of gallons in the container =  x
 x
∵ 4 gallons of water pouring out
∵ It is   full
 full
→ Multiply  by x to find the number of gallons left in it
 by x to find the number of gallons left in it
∴ The number of gallons left in the container =  x
 x
→ The difference between the numbers of gallons is 4 gallons
∵   x -
 x -  x = 4
 x = 4
→ Subtract the terms of the left side
∴  x = 4
 x = 4
→ Multiply both sides by 9
∴ 2x = 36
→ Divide both sides by 18
∴ x = 18
∵ x represents the full of the container
∴ The container can hold 18 gallons