Question 54697
The diameter of Earth is about twice that of Mars.
           E = 2M
:
 The diameter of Mars is about three times that of Pluto.
           M = 3d  &  E = 2M
Therefore:
           E = 2(3d)
           E = 6d
:
 Together, the sum of the diameters of Earth, Mars, and Pluto is about 30,000 kilometers less than the diameter of Uranus.
                 E + M + d = U - 30000
:
 If the diameter of Uranus is about 52,000 kilometers, what equation would be used to find the approximate diameter of Pluto? Let d = diameter of Pluto.

              d = U - 30000 - E - M
              d = 52000 - 30000 - E -M
              d = 22000 - E - M
:
Substitute 6d for E and 3d for M
              d = 22000 - 6d - 3d
              d = 22000 - 9d
         d + 9d = 22000
            10d = 22000
              d = 22000/10
              d = 2200