A person drove at 39 kmph for 3.5 hours, then increased his speed by 9 kmph, and reached the destination in another hour. Find the average speed (in kmph) of the person during the entire journey.

Correct option is C

Given:

Speed 1 = 39 kmph

Time 1 = 3.5 hours

Speed increase = 9 kmph

Time 2 = 1 hour

Formula Used :

Average Speed = $$\frac{\text{Total Distance}}{\text{Total Time}}$$​​

Distance = Speed $$\times$$​ Time

Solution:

Distance 1 = Speed 1 $$\times$$​Time 1 = 39 kmph $$\times$$3.5 hours = 136.5 km​

Speed 2 = Speed 1 + Speed increase = 39 kmph + 9 kmph = 48 kmph

Distance 2 = Speed 2$$\times$$Time 2 = 48 kmph $$\times$$1 hour = 48 km​​

Total Distance = Distance 1 + Distance 2 = 136.5 km + 48 km = 184.5 km

Total Time = Time 1 + Time 2 = 3.5 hours + 1 hour = 4.5 hours

Average Speed = $$\frac{\text{Total Distance}}{\text{Total Time}}$$​= $$\frac{184.5 \, \text{km}}{4.5 \, \text{hours}}$$​= 41 kmph

Therefore, the average speed of the person during the entire journey is 41 kmph.

Option (C) is right.