[attachment=5967]

Interesting graphs. From it I see that the speeds corresponding to the maximums of the 4 curves are, rougly:

200 m/s

350 m/s

500 m/s

1100 m/s

Just for curiosity, I draw in a graph the four inverse squared speeds with respect to the 4 gas' masses (132; 40; 20; 4. m on x axis and 1/v

^{2} on y axis): it's a quite perfect straight line (as it should be since kinetic energy is (1/2)mv

^{2} and has the same value for all 4 gases: being monoatomic = just traslational kinetic energy).

Edit: clearly that works because those 4 curves are symmetrical enough to approximate the average speeds with the speeds of the maximums.