A class grade is based on three tests each counting 15 percent, four quizzes each counting 10 percent and a homework notebook counting 15 percent, with the marks recorded across columns of a spreadsheet. Which of the following formulae would NOT be a correct calculation of the six-week weighted average for a student in row 2?
A=B2*0.15 + C2*0.15 + D2*0.15 + E2*0.1 + F2*0.1 + G2*0.1 + H2*0.1 + I2*0.15
B=(B2+C2+D2+I2)*0.15 + (E2+F2+G2+H2)*0.1
C=((B2+C2+D2+I2)*1.5 + (E2+F2+G2+H2))/10
D=(B2+C2+D2+I2)/15 + (E2+F2+G2+H2)/10 ✓ Correct
Correct answer: (D) =(B2+C2+D2+I2)/15 + (E2+F2+G2+H2)/10
Explanation
The weights are 15 percent for each of three tests and the homework, and 10 percent for each of four quizzes.
The first formula multiplies each test and the homework by 0.15 and each quiz by 0.1, which is correct.
The second formula groups the four fifteen-percent items and the four ten-percent items correctly.
The third formula multiplies by 1.5 and divides by 10, which equals the same 0.15 and 0.1 weights.
The fourth formula divides by 15 and 10 instead of multiplying by 0.15 and 0.1, giving wrong values.
Dividing a mark by 15 gives about 0.067 of it, not 15 percent.
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