What is the units digit for the number 123^(456) ?
How many noughts are at the end of these giant numbers?
Prove that if a^2+b^2 is a multiple of 3 then both a and b are multiples of 3.
Consider numbers of the form $u_n = 1! + 2! + 3!
+...+n!$. How many such numbers are perfect squares?