Tough and Tricky questions: Exponents.
If a and b are positive integers and x = 5^a and y = 4^b, which of the following is a possible units digit of xy?	Choose one:
A) 1
B) 4
C) 5
D) 7
E) 0
Only Units digit of X and Y contribute to the Units digit of XY.   Finding all values ofXfor positive values ofa   When,   a=1; X=5   a=2; X=25   a=3; X=125........ Units digit repeat in a cycle of 5......   Similarly, Finding all values ofYfor positive values ofb   When,   b=1; Y=4   b=2; Y= 16   b=3; Y= 64....... Units digit repeat in cycle of 4,6,4....   So, possible value of Units digit of XY can be found by considering all possibilities of Units digit of X and Y   When X=5 and Y=4: Units digit of XY = 20   When X=5 and Y=6: Units digit of XY = 30   Only one possibility for Units digit of XY i.e. 0.   therefore answer is 0
<result>E</result>
Tough and Tricky questions: Exponents.
If a and b are positive integers and x = 5^a and y = 4^b, which of the following is a possible units digit of xy?