You said that M is 13.7% of N
In algebraic notation M = 0.137 N
Divide each side by 0.137 : N = M / 0.137 = 7.3 M (rounded)
Online or in a real book, you can look up the radius of Mars.
Then write it in place of 'M' in this solution, and you'll have
an actual number for the radius of Neptune.
