If a is a 3-digit integer and b is a 3-digit integer, is the units digit of the product of a and b greater than 5?

(1) The units digit of a is 4.
(2) The units digit of b is 7.
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Target question: Is the units digit of the product of a and b greater than 5?

Given: a is a 3-digit integer and b is a 3-digit integer

Statement 1: The units digit of a is 4
There are several values of a and b that satisfy statement 1. Here are two:
Case a: a = 104 and b = 102. In this case the product ab = (104)(102) = ----8. So, the units digit of ab IS greater than 5
Case b: a = 104 and b = 100. In this case the product ab = (104)(100) = ----0. So, the units digit of ab is NOT greater than 5
Since we cannot answer the target question with certainty, statement 1 is NOT SUFFICIENT

ASIDE: When I write that the product ab = (104)(102) = ----8, I'm ignoring all digits in the product OTHER THAN the units digit, since that's all we care about.

Statement 2: The units digit of b is 7
There are several values of a and b that satisfy statement 2. Here are two:
Case a: a = 104 and b = 107. In this case the product ab = (104)(107) = ----8. So, the units digit of ab IS greater than 5
Case b: a = 100 and b = 107. In this case the product ab = (100)(107) = ----0. So, the units digit of ab is NOT greater than 5
Since we cannot answer the target question with certainty, statement 2 is NOT SUFFICIENT

Statements 1 and 2 combined
Statement 1 tells us that a = --4
Statement 2 tells us that b = --7
So, ab = (--4)(--7) = -----8
So, the units digit of ab IS greater than 5
Since we can answer the target question with certainty, the combined statements are SUFFICIENT

Answer:

Cheers,
Brent
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Brent Hanneson – Founder of gmatprepnow.com

If a is a 3-digit integer and b is a 3-digit integer, is the units digit of the product of a and b greater than 5?

Unit digit of A*B>5
Let x,y = unit digit of a and b

(1) The units digit of a is 4.
If y=2, x*y= 8
If y=1, x*y= 4
NS

(2) The units digit of b is 7
If x= 3, x*y = 21 ( unit digit is 1)
If x= 4, x*y = 28 ( unit digit is 8)
NS

St 1+2. Suff x*y= 28 ( unit digit is 8 )

Option C
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Never stop fighting until you arrive at your destined place - that is, the unique you. Have an aim in life, continuously acquire knowledge, work hard, and have the perseverance to realise the great life. A. P. J. Abdul Kalam

We are given that a is a 3-digit integer and b is a 3-digit integer and need to determine whether the product of a and b is greater than 5.

Statement One Alone:

The units digit of a is 4.

Statement one alone is not sufficient to answer the question. For example, if the units digit of b is 1, then the units digit of the product of a and b is 4, which is less than 5; however, if the units digit of b is 2, then the units digit of the product of a and b is 8, which is greater than 5.

Statement Two Alone:

The units digit of b is 7.

Statement two alone is not sufficient to answer the question. For example, if the units digit of a is 1, then the units digit of the product of a and b is 7, which is greater than 5; however, if the units digit of a is 0, then the units digit of the product of a and b is 0, which is less than 5.

Statements One and Two Together:

Using the information from statements one and two, we see that since the product of the units digits of a and b is 4 x 7 = 28, the units digit of the product of a and b is 8, which is greater than 5.

Answer: C
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