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.**