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
The OA is the option C.
How can I find the correct answer here? Experts, can you show me how would you solve this PS question?
Thanks in advanced.
The product of all the prime numbers
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The numbers are very spread apart (each answer choice is 10 times greater than the next answer choice). This means we can be quite AGGRESSIVE with our estimation.VJesus12 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
One approach:
We have the product (2)(3)(5)(7)(11)(13)(17)(19)
Let's see if we can group the numbers to get some approximate powers of 10
First (2)(5)=10, so we get (2)(3)(5)(7)(11)(13)(17)(19) = (10)(3)(7)(11)(13)(17)(19)
Next, 11 is close enough to 10, so we get: (10)(3)(7)(11)(13)(17)(19) = (10)(3)(7)(10)(13)(17)(19) [approximately]
Next, (7)(13)=91, which is pretty close to 100.
So we get (10)(3)(7)(10)(13)(17)(19) = (10)(3)(100)(10)(17)(19) [approximately]
Finally, 3(17)=51, and (51)(19) is very close to (51)(20), which is very close to 1000
So,(10)(3)(100)(10)(17)(19) = (10)(1000)(100)(10)= 10,000,000 [approximately]
Since 10,000,000 = 10^7, the best answer is C
Cheers,
Brent
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- Scott@TargetTestPrep
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2 x 3 x 5 x 7 x 11 x 13 x 17 x 19VJesus12 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
The OA is the option C.
How can I find the correct answer here? Experts, can you show me how would you solve this PS question?
Thanks in advanced.
Let's group some of these numbers to get powers of 10:
2 x 5 = 10 = 10^1
7 x 11 x 13 = 1001 ≈ 10^3
Now we are left with 3, 17 and 19:
3 x 17 x 19 = 51 x 19 ≈ 50 x 20 = 1000 = 10^3
Thus, the product of all the prime numbers less than 20 is closest to 10^1 x 10^3 x 10^3 = 10^7.
Answer: C
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