Same way you did on the calculator? =(100+85+75+96)/4, or with your values
in say B2:E2, then in F2 enter =SUM(B2:E2)/4. If you want it displayed as %,
devide by a further 100, format cell as percentage, 1 decimal point -
=SUM(B2:E2)/400, or =(100+85+75+96)/400, or else enter the values as
percentages 1, 0.85, 0.75 and 0.96, and then devide by 4 at the end