Math problem: need help

This question is from official guide 12th edition. Q> The product of all the prime numbers less than 20 is closest to which powers of 10? (A) 10^9 (B) 10^8 (C) 10^7 (D) 10^6 (E) 10^5 Ans : C The explanation shows we have to multiply all the primes but that takes too long. Isn't there more logical an...

Hello, I may not be replying to your question, but I have something to ask you. You have a very nice score. I am also going to start preparing for GMAT and this is my first post on this forum. I want to ask you, what study materials did you use? I mean books, forums, websites, etc. Please give me so...