The height of the pine is 20 meters. A snail crawls along it, every day it rises 2 meters up and every night it goes down 1 meter.

The height of the pine is 20 meters. A snail crawls along it, every day it rises 2 meters up and every night it goes down 1 meter. How many days will it take for a snail to rise to the top of a pine tree from the very beginning?

It must be understood that on the first day the snail began its movement from the zero mark. If it rises by 2 m in a day, and falls by 1 m during the night, then on the 1st day it has advanced:
0 + 2 – 1 = 1 m;
2nd day already:
1 + 2 – 1 = 2 m, etc.
18th day:
17 + 2 – 1 = 18
19th day:
18 + 2 = 20 m the snail will reach on day 19 and at night it will go down 1 m;
Answer: on day 19, the snail will reach the top of the tree 20 m.