15 downloads for $19.75 is stated this way in equation form:
15*(cost of each download) + (Registration Fee) = $19.75 or:
15*(x) + F = 19.75 or 15x + F = 19.75
In a similar way, 40 downloads for $43.50 is stated:
40(x) + F = 43.50
Where the "cost of each download"is x and where the registration fee is F and both are the same for both Bob and Jim.
Treat those two equations as a system of equations and solve for both x and F and you will get your answer.
Comment if you need more help.
(Just to give you a ball park figure, the cost per download is not over a dollar.)