Question on gravitation(low-altitude satellite)

[karis]

Homework Statement

A low-altitude satellite near the Earth's surface has a speed of 7.90 km s^-1. The radius of the Earth is about 4 times that of the moon and the ratio of the average density of the Earth to that of the moon is about 5:4. The speed of a low-altitde satellite near the moon's surface would be

A. 1.77km s^-1
B. 2.21km s^-1
C. 2.47km s^-1
D. 3.57km s^-1
E. 4.42km s^-1

Homework Equations

v² = GM/r

The Attempt at a Solution

i got the ans of 3.12km s^-1
the ratio of the mass of Earth to moon would be 80:1
n the ratio of radius of Earth to moon is 4:1
so i guess it's 7.9²/20 ?

Pls help, i don't know what i did wrong :(
Thz so much

 

[Fightfish]

Look at your equation again. What variable does 7.9^2/20 represent?

 

[karis]

7.9 is the speed of satellite near Earth's surface, and 20 is the ratio of the mass and radius combining, (1/80)*4

 

[Fightfish]

Not the individual terms, what variable does your answer aka 7.9^2/20 = 3.12 represent? Look carefully; is it the speed of the satellite?