Forty minutes is two-thirds of an hour. A fraction is found by setting up a ratio between a whole and its parts. The denominator, is the total number of parts, while the numerator, indicates how many parts are involved.
In this instance, the whole is one hour, which is equivalent to 60 minutes; therefore, the fraction is 40/60. To make the answer more useful, it should be simplified. To do this, find the largest number that is a factor of both the numerator and the denominator. The number 20 divides evenly into both 40 and 60, leaving the fraction 2/3.