(P.S. this should be in the math section not in the English one)
The gas ate three quarters of the Mohicans, so they ate 3/4 of 7 kilograms. To know how much they ate, you need to multiply the total number of Mohicans by the proportion ate:
7*(3/4) = 21/4 = 5.25
So they ate 5.25 kilograms from the 7 kilograms.
Hope this Helps! :)