Answer: The required percent of cost that was spent on books is 20%.
Step-by-step explanation: Given that a student's cost for last semester was $2100 and she spent $420 of that on books.
We are to find the percent of last semester's college costs that was spent on books.
The total cost of last semester, C = $2100
and
the cost of books in the last semester, S = $420.
Therefore, the percent of last semester's college costs that was spent on books is given by
Thus, the required percent of cost that was spent on books is 20%.