How to solve this question?
See “Help Us Test Out Our Quant Questions - An Experimental Quiz 1” in Non-Sequenced Quant Classes - GregMat. Around the 51 minute mark.
As you already know that
C = a*b
and even that unit digit of C should equal to either a or b.
if you multiply odd number with 5 you will get 5 as a unit digit 5
5 * 1 = 5 unit digit 5
5 * 3 = 15 unit digit 5
5 * 5 = 25 unit digit 5
5 * 7 = 35 unit digit 5
5*25 = 125 unit digit 5
I hope this will make it clear
What about 0?
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.