Question 1176443
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The other tutor has provided a good solution using the standard formal algebraic method for solving problems like this.<br>
This, and a large number of other types of problems, can be seen as a weighted average problem, or a mixture problem.  Problems like that can be solved by seeing where the weighted average lies between the two given averages.<br>
Consider the overall (weighted) average and the averages of the boys and girls on a number line: 68, 70.25, and 73.<br>
With simple arithmetic, determine that the class average lies 2.25/5 = 9/20 of the way from 68 to 73.<br>
That means 9/20 of the students in the class are girls.<br>
9/20 of 40 is 18, so there are 18 girls in the class; that means 22 boys.<br>