Answer:
Your average speed must be 90 mph in the remaining distance in order to have overall average speed of 60 mph
Step-by-step explanation:
The rule of distance is d = v × t, where
∵ You wish to average 60 mph on a drive from your home to town
∵ The distance from your home to town is 78 miles
→ Find the time of your journey using the rule above
∴ 78 = 60 × t
→ Divide both sides by 60
∴ [tex]\frac{78}{60}[/tex] = t
∴ t = [tex]\frac{13}{10}[/tex] hours
∵ You have averaged only 45 mph in the first 39 miles (half way)
∴ v1 = 45 mph
∴ d1 = 39 miles
→ Find the time of this part
∴ 39 = 45 × t1
→ Divide both sides by 45
∴ [tex]\frac{39}{45}[/tex] = t1
∴ t1 = [tex]\frac{13}{15}[/tex] hour
→ Subtract the time of the first part from the time of the journey to
find the time for the second part
∵ t2 = t - t1
∴ t2 = [tex]\frac{13}{10}[/tex] - [tex]\frac{13}{15}[/tex]
∴ t2 = [tex]\frac{13}{30}[/tex] hour
→ Now let us find the average speed of the second part of his journey
∵ d2 = 39 miles ⇒ (78 - 39)
∵ t2 = [tex]\frac{13}{30}[/tex] hour
∴ 39 = v2 × [tex]\frac{13}{30}[/tex]
→ Divide both sides by [tex]\frac{13}{30}[/tex]
∴ 39 ÷ [tex]\frac{13}{30}[/tex] = v2
∴ 90 = v2
∴ v2 = 90 mph
∴ Your average speed must be 90 mph in the remaining distance in
order to have overall average speed of 60 mph
