Rearrange the parts to make a meaningful sentence.

P. was not the boy's answer

Q. he had worked it out

R. but the speed with which

S. what surprised the teacher

Explanation

1. 01

   A "what" clause can be a subject: S "what surprised the teacher" needs a verb, and P gives it: "was not the boy's answer".

2. 02

   "Not ... but" is a pair, so R follows with "but the speed with which", and "with which" needs a clause: Q.

3. 03

   So the order is SPRQ: "What surprised the teacher was not the boy's answer but the speed with which he had worked it out."