Question

Suppose that the division N/5 leaves a remainder of 4, and the division N/2 leaves a remainder of 1, what is the ones digit on N?

Answer 1

Since N/2 leaves a remainder, N must be odd and ends with 1, 3, 5, 7, or 9.

N/5 also leaves a remainder, so N is not divisible by 5, so it does not end in 5.

The only correct choice is then 9, since

1 = 0•5 + 1 and 1 = 0•2 + 1

3 = 0•5 + 3 and 3 = 1•2 + 1

7 = 1•5 + 2 and 7 = 3•2 + 1

9 = 1•5 + 4 and 9 = 4•2 + 1

Alternatively, the given information is equivalent to saying

$N\equiv 4\pmod5\\\\N\equiv1\pmod2$

Then you can use the Chinese remainder theorem to find N.