Sample mAs problems 

mA = milliamps     s=time in seconds 

mA   x     seconds =  mAs 

SO   

1/5 of a second with 150 milliamps = ???? mAs

1/5    =   .2 milli seconds  SO   .2 x 150 = 30 mAs

Now you try to solve for these examples: 

1/10 of second at 200 mA 

1/5 of a second at 400 mA 

1/5 of a second at 100 mA

did any come out to the same mAs ?   

This is law of reciprocity-   Duplication of the exact same amount each time. We can have multiple combinations with mA and time to get the same mAs. mAs is the quantiy of our radiation.