Units digit problem - is MGMAT answer key wrong?

[leong_edwin]

In the advanced MGMAT quant book, I came across the following problem:

What is the units digit of y?

1) The units digit of y^2 is 6
2) The units digit of (y+1)^2 is 5

Answer key says B is right answer.


I agree that A and D are incorrect, due to fact that y can be either 4 or 6 for statement 1.

But for statement 2, we know that (y+1) needs sum to a number that has a ones digit of 5. Doesn't this mean that y can be either 4 or -6, which will both lead to either 5 or -5, which when squared will be 25. Doesn't this mean that the ones digit for y could be 4 or 6, so the answer should be E?

Any one have any insight on this?

[jnelson0612]

In the advanced MGMAT quant book, I came across the following problem:

What is the units digit of y?

1) The units digit of y^2 is 6
2) The units digit of (y+1)^2 is 5

Answer key says B is right answer.


I agree that A and D are incorrect, due to fact that y can be either 4 or 6 for statement 1.

But for statement 2, we know that (y+1) needs sum to a number that has a ones digit of 5. Doesn't this mean that y can be either 4 or -6, which will both lead to either 5 or -5, which when squared will be 25. Doesn't this mean that the ones digit for y could be 4 or 6, so the answer should be E?

Any one have any insight on this?

Wow, I can't disagree with anything you've said and think you are right. The problem should have stipulated that y is positive.

[parleybaldwin]

Perhaps when referring to "units digit," we are to assume that it is a positive number. Or maybe MGMAT just made that assumption. It could just be one of the quirks of the GMAT that we have to learn. Stupid test!...lol

[jnelson0612]

Perhaps when referring to "units digit," we are to assume that it is a positive number. Or maybe MGMAT just made that assumption. It could just be one of the quirks of the GMAT that we have to learn. Stupid test!...lol

Well, a digit itself is always positive, thus the units digit of -15 is 5. I still think we have trouble here--hard to say!