A reading specialist tests the reading speed of children in four ninth-grade English classes. There are 42 students in class A, 35 in class B, 33 in class C, and 39 in class D.
The mean reading speed in words per minute for the classes were as follows: class A, 220; class B, 185; class C, 212; and class D, 172. What is the mean reading speed for all classes combined?