Dear Candidate,

We know that, the rate of effusion is inversely proportional to molecular mass, i.e.

r1 / r2  = √ ( M2 / M1 )

And, the rate of effusion is ratio of the volume effused to the tima taken

i.e.  V1 * t2  / V2  * t1  = √ ( M2 / M1)

Since , the volume remains same we have

√ ( M2 / M1) = 3 and hence solving we get M2 = 36.

Hope it helps.