Weighted Average help please???? [Thank you!!!]

Hi-- Could someone please help me with this? I am confused. Please explain so I can follow you. Thanks to all! =]

Suppose Elizabeth is trying to calculate her grade in a math class. The course syllabus says that grades are based on the following breakdown: Homework, 20%; attendance, 10%; unit exams, 40%; and final exam, 30%. Elizabeth's totals are as follows: Homework, 85; attendance, 75; unit exams, 83; and final exam, 78. Calculate Elizabeth's overall (weighted) grade.

i'll put in parenthesis so it easier to visualize

(.2 * .85) + (.1 * .75) + (.4 * .83) + (.3 * .78) = 0.811

81.1%

Homework: 85 @ 20%
Attendance: 75 @ 10%
Unit exams: 83 @ 40%
Final exam: 78 @ 30%

Overall grade = 85 * 0.2 + 75 * 0.1 + 83 * 0.4 + 78 * 0.3
. . . . . . . . . . . . . = 17 + 7.5 + 33.2 + 23.4
. . . . . . . . . . . . . = 81.1