Answers:
The equation is L = 2D+52
After 39 days, the total length of the road is 130 miles
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How I got those answers:
D is the number of days
For each day, 2 miles is added on.
After 1 day, 2*1 = 2 miles is added on
After 2 day, 2*2 = 4 miles is added on
After 3 days, 2*3 = 6 miles is added on
After 4 days, 2*4 = 8 miles is added on
and so on...
In general, 2*D or 2D miles is added on after D number of days have passed by. For example, if 10 days pass by then 2*D = 2*10 = 20 new additional miles are added on.
The expression 2D is added onto the starting amount of 52
Overall the expression 2D+52 represents the total length of the road, which is L.
So L = 2D+52 is our equation
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Once we have the equation, we plug in D = 39 and use the order of operation PEMDAS to simplify
L = 2*D + 52
L = 2*39 + 52
L = 78 + 52
L = 130
After 39 days, the total length of the road is 130 miles