You're assuming that this data set represents all of Mrs. Hassaan's pupils. Certain students could be missing from the histogram - for instance, there could be one or two students who missed the test, but for certain reasons will be taking a make-up exam. Their grade would not be recording in this data set, but they still contribute to the class size, and so would be unaccounted for here.
I hope you know what '!' means
2!=2
3!=6
4!=24
basicalkly times every natural number including and before that number
nCr=

so

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This is how you would graph it
hope this helps :3
the image is below by the way X3