A ralload crew can lay 5 miles of track each day. They need to lay 135 miles of track. The length, $$ L $$ ( In miles), that is lieft to lay after d days is given by the following linction. $$ L(d)=135-5 d $$ answer the following questions. (a) How many days will it take the crew to lay all the track? $$ \square $$ thys (b) How many miles of track does the crew have lett to lay after 18 days? $$ \square $$ miles

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1. For part (a), we need to find the number of days $$ d $$ for which all the track is laid, that is, when  
   $$
   L(d) = 135 - 5d = 0.
   $$
   Solve for $$ d $$:
   $$
   135 - 5d = 0 \quad \Rightarrow \quad 5d = 135 \quad \Rightarrow \quad d = \frac{135}{5} = 27.
   $$
   So, it takes $$ 27 $$ days for the crew to lay all the track.

2. For part (b), we want to find out how many miles of track are left after $$ 18 $$ days. Substitute $$ d = 18 $$ into the function:
   $$
   L(18) = 135 - 5(18).
   $$
   Calculate:
   $$
   L(18) = 135 - 90 = 45.
   $$
   Therefore, after $$ 18 $$ days, $$ 45 $$ miles of track remain to be laid.
(a) It takes 27 days for the crew to lay all the track. (b) After 18 days, there are 45 miles of track left to lay.

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