Calculate the DC base current of a transistor circuit if the DC collector current is 4.9mA and the DC emitter current is 5mA?

Emitter current can be summed up by the formula:

IE = IC+ IB

Where IE = Emitter Current, IC = Collector Current, and IB = Base Current

So simply take IE - IC and you'll get an IB of about 100uA. (5mA-4mA = .1mA or 100uA)

Also, remember that IB controls IC and IE, not the other way around. You can also determine IB if Beta is known by the formula:

IC = IB * Beta

Which can be manipulated to equal IB = IC/Beta

Hope this was helpful.