If Nathan does 9 minutes of push ups for 3 days, that's 9 divided by 3, which is 3. (9/3=3) So nathan does 3 minutes of push ups each day, so just multiply each day by 3 to find the correlation between the two.
The Table should look like this:
# of days # of Minutes 1 3 2 6 3 9 4 12 5 15 6 18 7 21