"Smart" science student. Dear Tan Sok Yin

This girl in the pic is Sok Yin.

She can do calculus well, but not simple Mathemathics.
She bought ala carte mc bites which cost her RM 8, and also a milo which costed her RM 4.75.
Total=rm 12.75 (exclude tax)
But, she bought a SET of mc bites, which included Soft drink, French fries, and mc bites, will only cost RM9.
Smart right!? Science students.