bit o'math

Here's a little math problem for you--

Suppose you have a perfect score of 100 on all the homework exercises (worth 15% of your course grade), a 100 on your workout assignments (also worth 15% of your course grade), and a 9 on the 10% of your course grade given for class participation (being there, being prepared, staying on page, asking good questions, giving decent answers--generally making the class experience a pleasant and productive one for me and your classmates), what grade average on your examinations (altogether worth 60% of your course grade) do you need to achieve to earn an A in the course?

In other words, solve for X:

90 = .15 (100) + .15 (100) +.1 (9) + .6X

Whelk? (Actually whelk egg case)