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Question

Kelly walked from home to school. For the first 3 minutes, he was walking at an average speed of 40m/min. When he realized that he was going to be late by 10 minutes, he increased his speed by 10m/min. As a result, he was late by 2 minutes. Find the distance between his home and school.

Answer

3 x 40 = 120
40 x (t + 10) = 50 x (t + 2)
t = (400 – 100)/(50 – 40) = 30

40 x (30 + 10) + 120 = 1720
or
50 x (30 + 2) + 120 = 1720

Ans : 1720 m.

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After the first 3 minutes, he realised he was going to be 10 minutes late. By increasing his speed, he became only 2 minutes late. Thus, increasing his speed by 10m/min saved him 8 minutes. 

40*8 = 320

The extra 320m he would have covered in the 8 minutes, was covered by the extra 10m/min in his speed. Thus, 

320/10 = 32

His travelling time at 50m/min was 32 minutes. To get his total distance, 

32*50 + 3*40 = 1720m

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