3/5 = blue shirts
5/8 of 3/5 = on sale blue shirts
1/3 of 5/8 of 3/5 = medium on sale blue shirts
‘Of’ can be replaced by multipication symbols.
1/3 • 5/8 • 3/5
1/3 • 3/5 = 3/15 (or 1/5)
3/15 • 3/5 = 9/75 (or 3/25)
9/75 of the shirts are medium, on sale, and blue. If you wanted to simplify this it would just be 3/25 of the shirts.