To celebrate a colleague's graduation, the m coworkers in an office agreed to contribute equally to a catered lunch that costs a total of y dollars. If p of the coworkers fail to contribute, which of the folloing represents the additional mount, in dollars, that each of of the remaining coworkers must contribute to pay for the lunch?
The answer is py/m(m-p)
I tried to substitute numbers in place of the variables but couldn't come up with the above answer.