Mittens are marked down 25% the sale price is 3.15 a pair what was the original price of a pair of mittens

$12.6 -- 25% of 12.6 is exactly $3.15

I did this by taking 3.15 / 0.25

4.20   
when I do percentages at the store with sales, I always take whatever the percent off is, off the dollar amount.. so 25% would be to take 25 cents off every dollar. 

The volume of the water in the pool is (24.5 x 13 x 4) cubic feet.

The number of gallons is (24.5 x 13 x 4 cubic feet) x (7.48 gal/cu-ft)

80% means 0.80 .

To fill the pool to 80% of capacity, it takes

         (0.8) x (24.5 x 13 x 4 cubic feet) x (7.48 gal/cu-ft) gallons.