Total = 3^x, where x is the number of jobs worked.
Let x = 7th job:
Total = 3^7 = 2187 pennies. = $21.87

It looks like your first selection (a) is appropriate.
You can use the factors of the volumes 24, 27 and 48:
For example:
8 by 3 by 1 is a total volume of 24
or if you know that 4 times 2 is 8:
4 by 2 by 2
and so on
Here’s the answers hope this helps.