two children are at opposite end of an aluminium rod.one strikes the end of the rod.find the ratio of times taken by sound wave in air in aluminium to reach the second child.(given: speed of sound in air=346m/s; speed of sound in aluminium=6420m/s).
a dice is thrown 5 times a random find the probability that any one definite number come 4 times of upward and anyone different number comes at least 4 times a upward