Simon's water cooler can hold up to 4.2 gallons of water in it. simons fills the cooler so that there is water in all but 25% of the cooler. how much more water would simon need to add to fill the entire 100% of the cooler? a 1.05 gallons b 3.95 gallons c 3.15 gallons d 1.7 gallons

Based on the information given we know that 75% of the cooler is filled, because 25% is left. We only need to know what 25% of 4.2 is though, because the question is asking how much more he would need. 
25% of 4.2 is equal to .25*4.2 which equals 1.05. 
So in this scenario, the correct answer would be A.

Hold on lemme do the work