Explanation:
Let x be the number of pages that were skimmed through.
Time taken to skim through = 10 * (x/15) = 2/3 x min.
Time taken to read through = 15 * [(275-x)/10] = 1.5(275-x) min.
We have 2/3 x + 1.5(275-x) = 350.
Therefore 412.5 - 5/6 x = 350 and x = 75.