Answer: He needs to answer 20 mails.
Explanation:
Given : Mr. Kelly answered seven emails. This is 35% of the emails he must answer.
We can write 35% as 0.35. [To convert percent into decimal , we divide it ny 100.]
Let x be the number of mails that must be answered.
Then, according to the question we have,

Divide both sides by , we get

Hence, he needs to answer 20 mails.