Of the 65% who study either time, 47% study on weekends. That means 65%-47% = 18% study on weeknights only.
The fraction who study on weeknights is the total of the fraction who study on weeknights only (18%) and the fraction who study on weeknights and weekends (42%). That total is ...
18% +42% = 60% . . . . fraction who study on weeknights
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It can be helpful to draw yourself a Venn diagram.
