Question

8. Grandpa walks 1.5 km in 1 hour 20 minutes. Express his speed in m/sec. Also find how long would it take him to go from home to post office and return home, if the distance between home and post office is 150 metres.
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Answer 1

First problem-

Distance = 1.5km

$in \: metre = 1.5km \times 1000 \\ 1500m$

Time taken= 1hrs 20min (60+20 = 80min)

$in \: sec = 80 \times 60 = 4800sec$

Speed of grandpa is as follows-

$speed = \frac{distance}{time} \\ s = \frac{1500}{4800} = \frac{15}{48} \\ speed = 0.3125 {m}^{sec}$

Speed= 0.3125m/sec

Second problem-

Distance = 150m

Speed= 0.3125m/sec.

$time = \frac{distance}{time} \\ t = \frac{1500}{3125} = \frac{300}{625} = \frac{12}{15} = \frac{4}{5} \\ t = 0.8sec$

Therefore, time taken by grandpa to reach post office is 0.8 sec.

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Answer 2

Given:

Grandpa walks 1.5 km in 1 hour 20 minutes.

Distance between home and post office is 150 metres.

​ To find :

The speed of grandpa and the time taken to go from home to post office and return home,

Formula to be used:

$Speed = \frac{Distance}{Time}$

Solution:

Step 1 of 2:

Grandpa walks 1.5 km in 1 hour 20 minutes.

Distance = 1.5km

Distance in meter is, 1.5 × 1000 = 1500 m

Time taken is, 1 hour 20 minutes. = 80 minutes

Time in seconds is, 80 × 60 = 4800 sec

$Speed = \frac{Distance}{Time}$

$Speed = \frac{1500}{4800}$

$Speed = \frac{15}{48}$

Speed = 0.3125 m/sec

The speed of grandpa is 0.3125 m/sec

Step 2 of 2:

Distance between home and post office is 150 metres.

The time taken by grandpa to go from home to post office and return home is,

$Time = \frac{Distance}{Speed}$

Distance traveled from home to post office and return is, 150 + 150 = 300 m

Speed of grandpa is 0.3125 m/sec

$Time = \frac{300}{0.3125}$

Time = 960 seconds or 16 minutes

The time taken by grandpa to go from home to post office and return home is, 960 seconds or 16 minutes

Final answer:

Grandpa walks 1.5 km in 1 hour 20 minutes.

The speed of grandpa is 0.3125 m/sec

Distance between home and post office is 150 metres.

The time taken by grandpa to go from home to post office and return home is, 960 seconds or 16 minutes