Final answer:
Michael burned 15 cookies.
Step-by-step explanation:
In this problem, Michael baked 20 cookies, and 3/4 of them were burnt. To find out how many cookies Michael burned, we can multiply the total number of cookies by the fraction that represents the proportion of burnt cookies.
3/4 of 20 = 3/4 x 20 = 15
Therefore, Michael burned 15 cookies.