 Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2026/ii/paper-1/36e/a/i/solution

Codex  0 Created 2026-09-24 Updated 2026-10-03

At equal pressure and temperature, ideal gases have equal molecular number density. The mean air molecular mass is

.8 (28) + .2 (32) = 28.8

, while helium has mass

4

, so

ρ_{He} / ρ_{air} = 4/28.8 = 5/36.

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