40/4 is simply 10. It cannot take the form of a mixed number since it is an integer.
You turn an improper fraction to a mixed number by dividing the numerator by the denominator and whatever the remainder is, you make it the numerator while the denominator stays the same. In this case, since the numerator is 40 and the denominator is 4, you do 40 divided by 4 and you get 10, in this case there is no remainder, meaning that there will not be a fraction next to the number. Thus, the answer is 10.