Kangaroo

Kangaroo do 10 jumps in 1 minute and then three minutes lie , then will make 10 jumps in 1 minute and 3 minutes lie, and so on. Find the shortest time in which kangaroo do 30 jumps.

Correct result:

t =  9 min

Solution:

$t=30\mathrm{/}10\cdot 1+2\cdot 3=9\text{ min}$

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