Asample of an unknown gas effuses in 11.2 min. an equal volume of h2 in the same apparatus at the same temperature and pressure effuses in 2.31 min. what is the molar mass of the unknown gas?

The  time of  effusion  of  a  gas  is  directly proportional to  the square  root   of  the  molar   mass
Let  the  unknown  gas to be  represented by y
therefore;
Ty/TH2=sqrt(y/H2)
11.2min/2.31min=sqrt(y/2)
square  both  side  to  remove  the  square  root   sign
(11.2^2 / 2.31^2) =y/2
23.5=y/2 
multiply  both  side by  2
y  is  therefore  47