suppose you got 20 as c —> thus 0 becomes the unit digit , what does it tells you about the unit digit of a and b . All possible combination of a and b that gives c = 20.
1 20
2 10
4 5
No, clear unit digit.
Now, if we got 5 as the unit digit of number than with guarantee we can say that either a or b unit digit must be 5.
Eg if c = 15 .
1 15
3 5
5 as the unit digit in all cases.
Eg 2 ) if c = 225
1 225
3 75
5 45
9 25
15 15
Either a or b will have 5 as a unit digit.