Answer
Because the velocity is inversely proportional to the molar mass, the tails on the helium are $\sqrt20/4$ = ~2.2 times as long as those for neon and $\sqrt 84/4$ = ~4.6 times as long as those krypton.
Work Step by Step
Because the velocity is inversely proportional to the molar mass, the tails on the helium are $\sqrt20/4$ = ~2.2 times as long as those for neon and $\sqrt 84/4$ = ~4.6 times as long as those krypton.
