You want to multiply the probability of each key for the two cars which is 1/5 × 1/4 because there are five keys. this gives you 1/20 when you multiply :)
Basically there are 180 days in a school year so if a school buss is late 24 times in 30 days it will be late 168 times in a school year not counting the last days which will be exactly 173 late buses in a whole year .