Question

A delivery van travels at a constant speed of 60 km/h to cover a certain route in 1 hour and 45 minutes. If traffic increases the time taken by 15 minutes, by what percentage must the van's average speed decrease to complete the route?

• A. 10.5%
• B. 12.5%
• C. 15%
• D. 20%

Answer 1

Answer:

B. 12.5%

The van's original speed is 60 km/h.

Original time:

$1\text{ hour }45\text{ minutes}=1.75\text{ hours}$

Distance covered:

$\text{Distance}=60\times1.75=105\text{ km}$

With traffic, the time increases by 15 minutes:

$1.75+0.25=2\text{ hours}$

New average speed:

$\frac{105}{2}=52.5\text{ km/h}$

Percentage decrease in speed:

$\frac{60-52.5}{60}\times100$
$=\frac{7.5}{60}\times100$
$=12.5\%$