STEP 1:
Add the fractions to figure how many hours he worked total
= 13/4 + 21/3 + 15/12
use common denominator of 12
= 39/12 + 84/12 + 15/12
= 138/12
simplify by 6
= 23/2 or 11 1/2 hours worked
STEP 2:
Multiply hours worked by $10.50 an hour
= 11 1/2 * 10.50
= 11.5 * 10.50
= $120.75 earned
ANSWER: Doug earned $120.75
Hope this helps! :)