Robert chops 40% of the pile of firewood in 3/4 of an hour.
Then, we would chop 80% of the pile of firewood in 2*(3/4)=3/2 hours.
where 3/2 = 1.5 hours
Also, he would chop the missing 20% in (3/4)/2=3/8.
The full work is for 3/8+3/2 =15/8 hours
Now, if one hour is paid $12.00, we can use the rule of three to know how much is paid for 15/8 hours:
1h-----------$12.00
15/8 h -------- $x
where x=(12.00*5/8)/1
x=22.50
Hence, Robert will be paid $22.50 to chop the entire pile of firewood.
