Sum of digits

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diehard_gmat: Junior | Next Rank: 30 Posts; Posts: 16; Joined: Fri Jan 28, 2011 12:36 am; Thanked: 1 times

Sum of digits

by diehard_gmat » Fri Jan 28, 2011 12:50 am

What is the sum of the digits of the decimal form of the product (2^2009 * 5^2011)?

A. 2
B. 4
C. 5
D. 7
E. 10

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Source: Beat The GMAT — Problem Solving | Fri Jan 28, 2011 12:50 am

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Anurag@Gurome: GMAT Instructor; Posts: 3835; Joined: Fri Apr 02, 2010 10:00 pm; Location: Milpitas, CA; Thanked: 1854 times; Followed by:523 members; GMAT Score:770

by Anurag@Gurome » Fri Jan 28, 2011 12:53 am

diehard_gmat wrote:What is the sum of the digits of the decimal form of the product (2^2009 * 5^2011)?

A. 2
B. 4
C. 5
D. 7
E. 10

The product = (2^2009)*(5^2011) = (2^2009)*(5^2009)*(5^2) = (10^2009)*(5^2) = 25*(2009 zeroes after 1) = 25000.... (2009 zeroes)

Hence, sum of the digits of the product = (2 + 5) = 7

The correct answer is D.

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