Graham's Law

One of my textbooks lists the following practice problem as an example of Graham's Law:

"A sample of Xe takes 75 seconds to effuse out of a container. An unknown gas takes 37 seconds to effuse out of the identical container under identical conditions. What is the most likely identity of the gas?"

It gives 4 choices: He, O2, Br2, and Kr.

I've triple checked my math, but I keep getting an outrageous answer, using Graham's Law (where Xe is B and the unknown is A):

Rate A / Rate B = sqrt(Molar Mass B / Molar Mass A)

37 seconds / 75 seconds = sqrt (131.29 g/mol) / Molar Mass A)

0.493 = sqrt (131.29 g/mol / Molar Mass A)

(0.493)^2 = (131.29 g/mol / Molar Mass A)

Molar Mass A = 131.29 g/mol / (0.493)^2

Molar Mass A = 131.29 g/mol / 0.243

Molar Mass A = 540.29 g/mol

My textbook lists "O2" as the correct answer. I know that if you multiply 131.29 by 0.243, you get 31.9 g/mol (essentially, O2). Where is my math wrong?