Restricting dimension in a textbox.

[Pulkit_Thukral]

I have a requirement that :

Among devices that were not operational this month ,how many were operational last month and how many were operational 2 months back.

Ex-

March month-Total Devices-100

                  Operational Devices-60

                  Non-Operational Devices-100-60=40

Now,

Among these 40 devices(Non operational in March month), How many device were operational in Feb month?

                                      How many devices were operational in Jan month?

My question is: How to restrict calculation of jan month and feb month device count among these 40 devices only?