ASTRONOMY FUNDAMENTALS - Chapter 6, Exercise 2 Solution
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Estimating the Combined Mass of the Other Seven Planets

PROBLEM
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Jupiter's mass is 317.8 times Earth's, and about 2.5 times the combined
mass of all other planets. Estimate the combined mass of the other
seven planets, expressed as a multiple of Earth's own mass.

SOLUTION
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Step 1 - express Jupiter's own mass in Earth masses (given directly):
    Jupiter's mass = 317.8 Earth masses

Step 2 - use the real relationship that Jupiter's mass is about 2.5
times the combined mass of all other planets:
    Jupiter's mass = 2.5 x (combined mass of other 7 planets)

Step 3 - solve for the combined mass of the other seven planets by
dividing both sides by 2.5:
    Combined mass of other 7 planets = Jupiter's mass / 2.5
    Combined mass of other 7 planets = 317.8 / 2.5
    Combined mass of other 7 planets ≈ 127.1 Earth masses

ANSWER: The combined mass of the other seven planets (Mercury, Venus,
Earth, Mars, Saturn, Uranus, and Neptune) is approximately 127.1 times
the mass of Earth.

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WHY THIS WORKS AS AN ANSWER

The chapter states Jupiter's mass is about 2.5 times the combined mass
of everything else - a direct ratio relationship. Since Jupiter's own
mass is already given as 317.8 Earth masses, finding the "everything
else" total just requires reversing that ratio: dividing Jupiter's
known mass by 2.5, the same multiplier that connects the two
quantities. The result, about 127 Earth masses spread across seven
planets, is still a substantial real total - but it remains dwarfed by
Jupiter's own single, real 317.8-Earth-mass total, underscoring just
how lopsided the real mass distribution in the Solar System actually
is.