Question 915937
To solve part b, all you need to do is substitute the n in either equation for 20.  So, if we use the first equation of 900 + 5n, we can replace n with 20:


900 + 5(20) ------> 900 + 100 -----> 1000


Therefore, they will be 1000 feet when they meet.  You can verify by replacing n for 20 in your second equation:


1240 - 12(20) -----> 1240 - 240 -----> 1000


Final Answer for Part b:  1000 feet