I used 75 only because I need the total count in the three circle collectively. One could add up the individual regions, in which case we will need to add the following terms:

[75-(b+c+5)] + [45 -(a+c+5)] + [90 – (a+b+5)] + a + b + c + 5 = Total

You will notice that if I combined the terms [75-(b+c+5)] and b+c+5 (terms in the end above), we would just get 75, and that is what I did in one shot.

75 + a + [45 -(a+c+5)] + [90 – (a+b+5)] = Total

They are both equivalent, essentially you are considering one of the full circles to begin with.

Gayatri says

Why don’t we also take “75 – ( b+c+5)”? Why is 75 taken on its own?

GMAT Quantum says

I used 75 only because I need the total count in the three circle collectively. One could add up the individual regions, in which case we will need to add the following terms:

[75-(b+c+5)] + [45 -(a+c+5)] + [90 – (a+b+5)] + a + b + c + 5 = Total

You will notice that if I combined the terms [75-(b+c+5)] and b+c+5 (terms in the end above), we would just get 75, and that is what I did in one shot.

75 + a + [45 -(a+c+5)] + [90 – (a+b+5)] = Total

They are both equivalent, essentially you are considering one of the full circles to begin with.