The astroid is 100,000,000 miles away.
It's traveling at a speed of 100,000 miles per day.
100,000,000 ÷ 100,000 = 1,000
1,000 days.
The following sequence shown up top is arithmetic sequence
What the composite function in the question is actually trying to explain is;
First of all, in the function 
, we will find a result by writing 
where we see the letter 
.
We will write this result in the 
function where we see the letter and we will get the result.
