Because the number is closer to 16, it can be rounded off to that. 15 31/40 is the same as 15.78. When the decimal is over .50 it is rounded up to the next whole number, but when it is under .50 it si rounded down to the nearest whole number. ex. since this one is over the .50, it's rounded up, but if it were 15.36 then it would be rounded down to 15. hope this helps :D
