How do I solve this?

Question

One morning, Justin drove 4 hours before stopping to eat lunch. After lunch, he increased his speed by 10 mph. If he completed a 500-mile trip in 9 hours of driving time, how fast (in miles per hour) did he drive in the morning?

Answer 1

Before lunch:

 

   D = r * 4

 

 

After lunch:

 

   500 - D  = (r+10)*5

 

Adding them together

 

500 = 4r + 5(r+10)

 

500 = 4r + 5r + 50

 

500 = 9r + 50

 

450 = 9r

 

r = 50

 

50 mph before lunch, 60 mph after lunch

 

Before lunch:  4*50 = 200 miles

After lunch:   5*60 = 300 miles