Mason is purchasing wooden blocks to build a birdhouse. Each block costs $1.80 and mason wants to spend less than $36.

If each block costs $1.80, then Mason can buy a maximum of 20 blocks with $36 ($36 / $1.80 = 20 blocks).

Therefore, Mason can buy up to 20 blocks with $36 but he wants to spend less than $36, so he can buy any number of blocks less than 20 to build a birdhouse.