Answer: The required number of machines is 24.
Step-by-step explanation: Given that a machine can stamp 40 envelopes in 8 mins.
We are to find the number of machines that are working simultaneously are required or needed to stamp 120 envelopes per minutes.
We will be using the UNITARY method to solve the given problem.
In 8 minutes, number of envelopes stamped by a machine = 40.
So, in 1 minute, the number of envelopes stamped by the machine is given by

Now, number of machines required to stamp 5 envelopes in 1 minute = 1.
So, number of machines required to stamp 1 envelope in 1 minute is

Therefore, the number of machines required to stamp 120 envelopes in 1 minute is given by

Thus, the required number of machines is 24.