If 4.5 buckets are needed to complete one wall, three walls will require 3*4.5=13.5 buckets.
At a price of $0.8 each, the buckets will total at $0.8*13.5=$10.8
It is also possible to calculate 0.8*13.5 in a different way. Because 0.8=8/10, we can subtract 2/10 from 135 and reach the same answer. The reason for doing this is the ease of calculating 1/10 of 13.5 which is 1.35
If 1/10 of 13.5=1.35, 2/10 of 13.5 is 2*1.35= 2.7
and now we subtract 13.5-2.7= $10.8