The first problem solution i will post in below figure the solution of this is 5mw.
the current flowing through r2 is 2mA and therefore the current flowing through the vA is 1mA. therefore the solution is vA multiplied with 1mA. i think this explanation is clear to you.
The 3rd one is apply source transformation at the input terminal then apply kvl then you will get the equation from that equation you can find the solution and it is 1.65mA. i think this may clear your question…
The second also use source transformation at the input then you will get the value of current. add this current and the current given in the circuit then you can get the over all current as 0.25mA. From this use current division rule you can get the short circuit current as 0.2mA. and find Req. that is to find by s/c the voltage source and o/c the current source then you will get the value is 25kohm. now we can easily implement the norton’s circuit. The s/c current and equivalent resistance are connected in parallel that gives the circuit. If have any doubt in explanation ask below.