I get D - Approximately 50%
Set up as follows:
Junior Rep visits the office 1/20 days.
Senior rep visits the office 1/30 days
(1 visit/20 days)*(365 days/year)*(2 years)= ~36 days visiting the office in a 2 year time span.
(1 visit/30 days)*(365 days/year)*(2 years)= ~24 days visiting the office in a 2 year time span.
36-24 = 12
12/24(100) = 50%
If you notice, when I set up the equation to get the number of days each rep visited in the two years, the "labels" canceled each other out. I remember learning about this technique in science class to make conversions (for example from hours to minutes to seconds). Essentially, you want the solution you're looking for (in this case the # of visits), to end up on top and alone. You'll notice that there is a "day" label on top and bottom - they cancel each other out. Also, you'll notice there's a "years" label on top and bottom as well, and they cancel each other out. This leaves "visits" alone and on top.
I hope that makes sense...and I hope that's what you were looking for...