Arnold K H Tan's answer to bubbletea's Secondary 4 E Maths Singapore question.

done {{ upvoteCount }} Upvotes
clear {{ downvoteCount * -1 }} Downvotes
Arnold K H Tan
Arnold K H Tan's answer
2090 answers (A Helpful Person)
1st
Just calculate as a simple ratio, just like you learn for map scales. You do have to, of course, memorise the powers for prefixes such as micro being 10^-6.