four boys step out from the same place.if their step measures 16cm,28cm,40cm,77cm respectively,at what distance from the starting point will they again step together?

Consider the boy whose each step measures 16 cm.
His first step will help him cover a total distance of 16 cm.
His second step will help him cover a total distance of 2×16 = 32 cm.
So, nth step helps him cover ‘16n’ cm of distance.
In other words distance covered by a boy is a multiple of the measure of his step.
Distance at which they all step together will have to be a common multiple of their steps measures. 
Hence we need to just find the LCM of 16, 28, 40 and 77.

LCM = 2×2×2×7×2×1×5×11 = 6160 
So the least distance at which they step together again is 6160 cm and then they will step together at a distance (in cm) which is any multiple of 6160.