Answer:
(Choice A) 5M
Explanation:
Let total no. of e-mails in inbox be x
Since she read 80 % of mails i.e 80 % of x
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So, she read 0.8x e-mails.
So, remaining email = total e-mails - read e-mails

Since we are given that Danette still has M unread e-mails.
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Thus the expression represent the number of e-mails Danette had in her inbox before she started reading is 5M.
Hence Choice A is correct.