So we are given that Logan read a total of 54 pages in total, math and geology. Next, we are given that out of the 54 total pages Logan read, 39 pages were geology.
We can simply find how many pages of math Logan read by subtracting the number of pages Logan read for geology (39), by the total number of pages he read (54).
54-39=15
Logan read 15 pages in the math textbook.