Question

Lastly, there is tunnel A can cross it in 1 Minute, B-2 min, C-4min,
D-5mins
Then there is only one torch light, at once nly 2 members can cross,
what is the process to all members go cross that tunnel.
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Answer 1

Its simple first D and A will go taking them 5 min and A will return making total trip of 6 min.
Similarly A and C will go where A will return again making their total trip of 5 min.
At last A and B will which will take them 2 min to cross the tunnel. So all will have crossed the tunnel in least time together. That total time will be 6+5+2 min= 13 min.