If four friends can eat four cakes in four days, then four friends will eat an average of one cake per day, so each friend will be able to eat 1/4 of a cake per day.So 10 friends will eat 10/4 of cake per day, or two and a half cakes per day, so they will eat 2,5 * 10 = 25 (twenty-five) cakes per 10 days, assuming they will eat at the same rate