Question 1a.
=VLOOKUP(D2,G$1:H$9,2,FALSE)

Question 1b.
=COUNTIF(results!B$2:B$13,A2)

Question 1c.
=SUMIF(results!B$2:B$13,statistics!A2,results!E$2:E$13)

Question 1d.
=RANK(C2,C$2:C$7,0)

Question 2a.
=($A5*(1-B$4)-$B$1)/$B$1

Question 2b.
(Price, Discount)
(80,10%), (90, 20%)
Remarks: Advanced users may use conditional formatting to format the cells with values between 20% to 30%.

Question 2c.
When there are more than one cell with formula referring to the same cell reference,
in case we need to change its value, we only need to make a single change to that cell reference.
(In contrast, we would need to make changes in every formula if we use a constant value.)

Question 3.
=IF(AND(A2>50, B2>50), IF(C2>50,"A","C"),IF(C2>50,"B","F"))

Question 4a.
X Y (Scatter) Chart (Scatter with Smooth Line)

Question 4b.
=COS((A2+180)*PI()/180)
Remarks: The parameter in the COS() function needs to be in radian, instead of degree.
You may have to convert it from degree to radian if the value in the question is degree.