Need help regarding margin of error.

[AceThePsychic]

The formulas we were given to solve these with all required a margin of error. I was wondering if I am supposed to use the 5 gallons figure as the margin of error, or if there is a way to calculate for it?

 

[katxt]

The MoE can be calculated using the 90%, the 10 and the unknown sample size. The answer is 5. Work backwards.