After reading 80% of her emails in her inbox, Danette still has M unread emails.

Answer:
[tex]\frac{M}{1-0.8}[/tex] and [tex]5M[/tex]
Step-by-step explanation:
1. Let's imagine that you have 100 e-mails unread. When you read 80% of these e-mails you have the following amount [tex]M[/tex] unread:
[tex]100-(0.8)(100)=20[/tex] e-mails
2. If you substitute 20 e-mails into the equations shown above, you will obtain the original amount:
[tex]\frac{20}{1-0.8}=100\\5(20)=100[/tex]
answer : option A and B
After reading 80% of her emails in her inbox, Danette still has M unread emails
M unread emails
Let x be the number of emails before reading
80% of emails are left in the box. So, 0.08x emails are left in the box
Now we make an equation
number of emails - 80% of unread emails= number of unread mails
x - 0.08x = M
Now we factor out x
x ( 1- 0.08) = M
So [tex]x = \frac{M}{1-0.8}[/tex]
or [tex]x = \frac{M}{0.2}= 5M[/tex]