Someone would tell you,

Think of a number,

add 3

multiply by 5,

subtract 2

multiply by 2

subtract 1

multiply by 4

subtract 100

divide by 40

The result is your original number.

Similar games are more mysterious in modular arithmetic.

Select any odd prime, p > 5

Do following in arithmetic mod p.

Find inverse of 3 mod p.

add (p+1)/2

divide by 5

add (p+1)/2

subtract from p

divide by 2

add (p+1)/2

multiply by 3

add (p+1)/2

result is 1.