According to a car dealer's report, 1/3 of the cars sold during a certain period were sedans and 1/5 of the other cars sold were station wagons. If N station wagons were sold during that period, how many sedans, in terms of N, were sold?
Since 1/3 of the cars sold during a certain period were sedans can we assume that there are only 2 types of cars and 2/3 of the cars sold during that same period were wagons. Likewise, can we assume that 4/5 of the other cars were sedans?
Nope, it is apparent that there were more than two types of cars sold during that period. Otherwise, it'd be 1/3rd sedans and 2/3rd wagons.
Proportion of other cars sold = 1- 1/3 = 2/3
Proportion of wagons = 2/3 *1/5 = 2/15.