Argon exists in three isotopic forms, 36Ar,

Question

38Ar, and 40Ar.

What is the relative rate of effusion of the
fastest to slowest, ufast/uslow?

Answer 1

The fastest will be Ar36; the slowest will be Ar40.
(rate 36/rate 40) = sqrt(40/36)

Answer 2

Grahams law, difusion rate is proportional to inverse sqrt of mass.

let the fastest rate be RATE

relative rate of second is RATE(sqrt(36/38)= .973 RATE

relative rate of last is
Rate(sqrt(36/40)=.949Rate

Answer 3

Please note that you didn't get too different answers here.
My response gives rate of fastest = 1.05 x slowest.

Bob P gave rate slowest = 0.949 x rate fastest.
Also note that 1/1.05(4) = 0.949