MathDB

On a natural number

n

you are allowed to operations :

(1)

multiply

n

2

(2)

subtract

3

from

n

. For example starting with

8

you can reach

13

as follows :

8 \longrightarrow 16 \longrightarrow 13

. You need two steps and you cannot do in less than two steps. Starting from

11

, what is the least number of steps required to reach

121

20