Rohit says 5/8 is bigger than 3/4, because the top is 5 and 5 is bigger than 3

Why

Rohit is comparing only the top numbers, which works when the bottom numbers are the same — 5/8 really is more than 3/8 — but here the bottoms differ (8 and 4), so the pieces are different sizes and you cannot compare the counts directly. The fix is to rewrite the fractions so they are cut into the same-sized pieces. 3/4 is the same as 6/8 (multiply top and bottom by 2), and now both are in eighths: 6/8 versus 5/8, so 3/4 (as 6/8) is the bigger one. Test it: picture two identical chocolate bars — one split into 8 squares with 5 taken, the other split into 4 pieces with 3 taken; three of the bigger pieces is more chocolate than five of the smaller ones. His sister is right: make the bottoms match first — 3/4 = 6/8 is bigger than 5/8 — the top number alone does not tell you when the bottoms are different.