A railroad crew can lay 5 miles of track each day. They need to lay 140 miles of track. The length, L (in miles), that is left to lay after d days is given by the following function.
L(d) = 140-50
Answer the following questions.

(a) How many days will it take the crew to lay all the track?

(b) How many miles of track does the crew have left to lay after 18 days?