chetan2u
RenB
If m and n are positive single-digit numbers such that the units' digit of m^2 and m^4 is the same and the difference between the units' digit of n^3 and n^4 is zero, then what is the units' digit of m+n?
(1) m = n
(2) m is divisible by 5
\(m^2\) and \(m^4 \) having same digits mean that m digit has a repetition after two consecutive powers or after every power.
Thus m could be 1,4,5,6,9.
\(n^3\) and \(n^4 \) having same digits mean a set of consecutive powers give same units digit, meaning n would give units digit equal to itself every time.
Thus m could be 1,5,6.
We are looking for m+n.
(1) m = n
The answer could be 1+1 or 5+5 or 6+6, that is possible answers are 2 or 5.
(2) m is divisible by 5
m is 5. Nothing about n
Combined.
m=n=5
Suff
chetan2u how did you deduce these statements? Did you try it out for a few numbers and then concluded it would be true for all such numbers?
1. \(m^2\) and \(m^4 \) having same digits mean that m digit has a repetition after two consecutive powers or after every power.
Thus m could be 1,4,5,6,9.
2. \(n^3\) and \(n^4 \) having same digits mean a set of consecutive powers give same units digit, meaning n would give units digit equal to itself every time.
I actually started computing the given powers for all numbers in the 1-9 to reach the answer, which was correct but it took me a ton of time to get to the answer. Any tips on how to improve speed in such types of questions?