Final answer:
The number of people receiving a free t-shirt at a concert, given there were m people attending, can be represented by the expression (3/4)m - 15, where (3/4)m calculates three-quarters of the attendees, and 15 is subtracted from that number.
Step-by-step explanation:
The student's question asks about turning a word problem into a mathematical expression, specifically related to the number of people receiving a free t-shirt at a concert. Given there were m people at a concert, and that fifteen less than three-quarters of these people will receive a free t-shirt, we can represent the number of people receiving a t-shirt with the expression (3/4)m - 15. This is the calculation of three-quarters of the people ((3/4)m) subtracting the fifteen people who will not receive a t-shirt (- 15).