In The Hitchhiker’s Guide to the Galaxy by Douglas Adams, the supercomputer ‘Deep Thought’ calculates the answer to the ultimate question of life, the universe, and everything. After 7.5 million years of computation, it delivers the answer: 42.

Math.

M is the 13th letter of the alphabet

A is the 1st

T is the 20th

H is the 8th

13+1+20+8=42.