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Bunuel
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BrentGMATPrepNow
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Since we are working with only the units digit, the approach will work fine for larger values as well.
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Bunuel
x, y, and z are consecutive positive integers such that x < y < z. If the units digit of x^2 is 6 and the units digit of y^2 is 9, what is the units digit of z^2?

(A) 0
(B) 1
(C) 2
(D) 4
(E) 5

Let's find some values that satisfy the given information.

If the units digit of x^2 is 6, then it could be the case that x = 6 (since 6^2 = 36)
Since x is the first consecutive integer, it could be the case that y (the second integer) equals 7.
Notice that y = 7 satisfies the condition that the units digit of y^2 is 9 (since 7^2 = 49)

Since x = 6 and y = 7 satisfy the given information, we now know that z COULD equal 8, in which case z^2 = 8^2 = 64
So, the unit's digit of z^2 must be 4

Answer: D

Is there a more scientific method for solving or proving this? If I try the values from 1-9 it quickly gets clear what we are after but what if the values would go far beyond 10 how would I solve it then?
Thanks a lot for a reply!
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Let's check the unit digit patterns:
4^2= 16-> ends in 6
6^2= 36-> ends in 6
So, the num's ending in 4 or 6 give squares ending in 6. So, the possible values of x have unit digits: 4 or 6.

Now for y^2 ending in unit digit 9.
3^2= 9
7^2 49-> ends in 9
So, the possible unit digits for y is 3 or 7.

Now, let's test it for consecutive integers: x,y,z.

Let's test x=14
x^2= 196-> ends in 6,
Then y=15, y^2= 225-> ends in 5 not valid.

So, let's try x=16, y=17, and z=18.
16^2= 256-> ends in 6
17^2= 289-> ends in 9
18^2= 324-> ends in 4

It works
So, answer is D.

Hope it helps
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