Since 40% of 90 is equivalent to 36, you have to subtract 90 and 36 to find out how many minutes he spent without cleaning, which would be 54 minutes that he spent not cleaning.
if he spent 40% of his time cleaning, that means he spent 60% of his time not cleaning and to find out how many minutes that is you find 60% of 90 minutes which is 90 x .6 which equals 54 minutes
