To find how much he used for both jobs, you should add the quantity used in the first job to the quantity used in the second job.
In the first job, he used 3/8 gallon, and for the second he used 1/3.
So w get the equation 3/8 + 1/3.
To add 2 fractions, they should have the same denominator.
So you multiply both numerator and denominator of the first fraction by the denominator of the second fraction 3, and you multiply both numerator and denominator of the second fraction by the denominator of the first fraction 8.
3*3/8*3 + 1*8/3*8 = 9/24 + 8/24 = 17/24.
So the painter used 17/24 gallon of paint for both jobs.
Hope this Helps! :)