Now consider a situation where two processes can write to the same file at the same time,

but not three.

Task $7(3$ Marks$).$ Suppose processes $P_1$ and $P_2$ are defined as in Task $6$ and a third

process, $P_3$ is defined by the steps:

After how many milliseconds will we have that all three processes are trying to write

to the same file at the same time?

Write down a system of congruences to express this fact.