GMATD11 wrote:

The product of all the prime numbers less than 20 is closest to which of the following powers of 10?

a) 10^9

b) 10^8

c) 10^7

d) 10^6

e) 10^5

2*3*5*7*11*13*17*19

10*21*143*17*19=

any easy way because i cnt do above multiplication in less time.

Thanks

OPEN DISCUSSION OF THIS QUESTION IS HERE: http://gmatclub.com/forum/the-product-o ... 35192.html The product of all the prime numbers less than 20 is closest to which of the following powers of 10 ?(A) 10^9

(B) 10^8

(C) 10^7

(D) 10^6

(E) 10^5

We should find the approximate value of 2*3*5*7*11*13*17*19 to some power of 10.

# of different approximations are possible.

Approach #1:2*5=10;

3*7=~20 (actually more than 20);

11*19=~200 (actually more than 200);

13*17=~200 (actually more than 200);

\(2*3*5*7*11*13*17*19\approx{10*20*200*200=8*10^6}\approx{10^7}\).

Answer: C.

Approach #2:2*5=10

3*17=~50 (actually more than 50);

7*13=~100 (actually less than 100);

11*19=~200 (actually more than 200)

\(2*3*5*7*11*13*17*19\approx{10*50*100*200}=10^7\).

Answer: C.

OPEN DISCUSSION OF THIS QUESTION IS HERE: http://gmatclub.com/forum/the-product-o ... 35192.htmlThe OA is marked as 'D' for this question. Why do we have 2 threads with same question? Is it to keep number of posts readable?