IMAT 2011 Question 75 Physics in Universal gravitation law

Hey, I might be a bit slow because I am just learning physics but this question was pretty much straight forward it didnt even require physics knowledge it was testing the candidate’s abilities in math and exponents basically. I solved it by hand and i got the answer to be A where 10 is to the power of 19.
I then checked it on a calculator and from the answer sheet and it turned out to be B not A, where 10 is to the power of 20. Then I understood that i had to move the decimal as I have completely disregarded the numbers and just worked with the exponents.

My question is, how am I supposed to solve this in the exam? Do i have to multiply and divide all the numbers or how am I supposed to know that it was 18.75 and i should move the decimal? Is there a strategy because otherwise I will always make the same mistake where i disregard the numbers and work on the exponents alone

Thank you in advance !!

Hope it helps :>