"Susan read 1/4 of a book on Monday. She read 3/10 of the same book on Tuesday. "
monday → 1/4 book
"She read 3/10 of the same book on Tuesday. "
tuesday → 3/10
"On Wednesday, she read 1/3 of the remaining pages."
remaining → 1 - 1/4 - 3/10 = 20/20 - 5/20 - 6/20 = 9/20
wednesday → 1/3 of remaining = 1/3 x 9/20 = 3/20
left → remaining - wednesday = 9/20 - 3/20 = 6/20
left → 156 pages
6/20 → 156 pages 1/20 → 26 pages 20/20 → 20 x 26 = 520 pages