**EXPERT ANSWER**

The time taken by the gas can be calculated by using

t= x^{2} /2D

Here x is distance travelled and D is diffusion coefficient

Here in this problem they are 4 and 1.78 × 10^{-5}

t = 16 /2(1.78× 10^{-5})

= 16/ 3.56× 2 ×10^{-5}

= 16/ 7.12×10^{-5}

= 16× 10^{5} / 7.12

= 3.88× 10^{5} seconds