The volumetric mean radius of Mars is about 3389.5 km. We know that it's 13.7% of Neptune's mean radius, so we can write it down as:
3389.5 = 13.7% of x (where x is Neptune's radius)
3389.5 = 13.7% x
3389.5 = 137/1000x / * 1000 (both sides)
3389500 = 137x / ÷ 137 (both sides)
x = 24740.87591240876
x ≈ 24740.88
Answer: The mean radius of Neptune is about 24740.88 km.
