1) Gathering the data
5 shelves =
1 shelf = 2 1/12 ft long
Juan bought an 18 ft long log
a) Since no other dimension but the length of each shelf let's divide the
2 1/2 feet = Converting that into an improper fraction
2 1/2 feet = 5/2
18 ÷ 5/2 = 18 x 2/5 = 36/5 or 7.2 ft
5 shelves = 36 ft long.
No, he doesn't have enough lumber to make 5 shelves.
He needs 18 ft more or another log.
b) The first way is dividing 18 ft by 2 1/2 ft (1 shelf length).
The second one is multiplying 5 by 2 1/2 ft and then compare both results.