Meryl needs to add enough water to 11 gallons of an 18% detergent solution to make

The percentage here must be assumed as by volume. Therefore, it means that the solution has 0.18 gallons of detergent / gallon of solution. If Meryl needs to add 11 gallons of water then she has 2.41 gallons of detergent. Calculation is as follows:

.18 = x / x+4

Solving for x, we obtain

x = 2.41