I've explained what the question is asking for and shown you how the calculation gives
the answer they want.
I admit the wording of the question can be misinterpreted.
I'm certain that, in this question, the whole point being tested is that you
- know g on the Earth's surface due to the Earth is approx 10m/s (You did)
- calculate correctly the value of the Moon's field strength at the Earth (you did)
- realize this is a "ball park" figure. (do you?)
I'll say it again.
If the one is 3 x 10
-5 (approx) and the other is 10 (approx) then the first one is
what the question has described as "about 3 millionths the value of" the second. 3 x 10
-6So the point of the question was that you correctly get that value of approx. 3 x 10-5
Think
Let's say the first value was 3 and the second was 1,000,000
What fraction is this?
Multiply both top and bottom by 10
-5you get
3 x 10-5 and 10
What is this fraction?
This is the fraction you are working out.
The ratio of the Moon's field to that of the Earth at that point.
Maybe you are getting mixed up between "1 in 3 million" and "3 millionths"? These are not the same.