Bob can cut through a log in two minutes. How long will it take Bob to cut a 30-foot log into 2-foot sections?

You have a log that is 20 feet long, and you are breaking it into 2-foot long sections.
We are cutting it into 10 equal pieces, each piece is 2 feet long (20 feet ÷ 2 feet = 10 pieces)
I am going to make a diagram where an equal sign, =, is a piece of the tree, and a slash, /, is a cut.

If we look at this, there are 10 pieces, and 9 cuts.
"Bob can cut through a log in two minutes."
This means that each of the 9 cuts takes 2 minutes, 9×2=18
It will take Bob 18 minutes to cut up the log