I am pretty sure it is (a) because 9 and 15 and 3 and 6 can be all divided by 3
p q
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| | s
| ----------------------------------------------------- r
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| | | -------------------------t
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q || t is not true as you can see
So since this chart shows which subject a certain number of students failed all of them would fall into the category of failing at least on subject so you just add up all the numbers
12+5+8+2+6+3+1= 37
5 5/10 = 5.5
4 2/10 = 4.2
Add 9.7