Hurry - rush

At an average speed 7 km/h I will come from the school to the bus stop for 30 minutes. How fast I need to go if I need to get to the bus stop in 21 minutes?

Correct result:

v =  10 km/h

Solution:

$v=7\cdot 30\mathrm{/}21=10\text{ km/h}$

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