It is given that there are 5 flags of different colours and each signal requires the use of two flags, one below the other. The upper vacant signal can have any one of the 5 flags, which means that there are 5 possible ways to occupy the upper vacant signal.
Since it is given that there is a requirement of two flags to make a signal, thus, if one flag has occupied the upper place, then there are only 4 flags left to obtain the lower space. So there are 4 ways to fill the lower vacant space of signal.
The total number of different signals that can be generated is calculated by the multiplication principle which states that if an event can occur in m different ways and follows another event that can occur in n different ways, thus the total number of occurrence of the events in the given order is
The different signals can be make as
Thus, there are 20 different ways to generate different signals.