One morning Justin drove 4 hours before stopping to eat lunc

One morning, Justin drove 4 hours before stopping to eat lunch. After lunch, he increased his speed by 10 mph. If he completed a 320-mile trip in 6 hours of driving time, how fast (in miles per hour) did he drive in the morning?

Solution

Let speed of Justin in the morning be X mph.

Therefore, we can write total distance as:

Total distance = Distance covered in first 6 hrs + Distance covered in last 2 hrs

320 = X*4 + (X+10)*2 (Since, Distance = Speed*Time)

320 = 4X + 2X + 20

6X = 300

X = 50 mph