Please take a look at the question I have attached here. For this, I found a wiki article for this formula that I have also attached here. However I found it complicated and didn't understand a word in the section I have drawn a rectangle over and underlined in light grey .
Please help me to solve this question..

Where are you having difficulty? The mole fraction of He is 2/5, and the mole fraction of O2 is 3/5, so the equation becomes:

\Delta S_{mix} = -5R(\frac 2 5 \ \ln(\frac 2 5) + \frac 3 5 \ \ln(\frac 3 5 ))

He he he... I guess I got confused by the complex notations in wiki. Thanks for making it clear.