When a class took a math test, 15% of the class failed, 25% made some mistakes (but didn’t fail), and 24 students got perfect scores. how many students were in the class?

15%+25%=40%, meaning that 40% of the class didn’t get a perfect score and 60% did. If 24 students got a perfect score and 24 students is equal to 60% of the class, you can solve by setting up a proportion. 
24 students X students——————- = —————-60% of class 100% of class
Next, cross multiply to get 2400 = 60X. Divide both sides by 60 and you’ll get 40. There are 40 students in the class. 
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The correct awnser is 29.76 pounds.