If a and b are positive integers and x = 4^a and y = 5^b, which of the following is a possible units digit of xy?  Pick
A) 0  B) 4  C) 5  D) 7  E) 8
The units digit of 4^a is either 4 or 6.   The units digit of 5^b is 5.   4*5= 
<gadget id="calculator">4*5</gadget>
<output>20</output>
20 and   6*5= 
<gadget id="calculator">  6*5</gadget>
<output>30</output>
30.   The units digit of xy is 0.   The answer is 0.
<result>A</result>
If a and b are positive integers and x = 4^a and y = 5^b, which of the following is a possible units digit of xy?