The Solution:
Given:

Let:
x = the number of calls received on the first evening.
y = the number of calls received on the second evening.
z = the number of calls received on the third evening.
So,

So, we have:

Substitute for x.

Divide both sides by 6.

To find x.

To find z:

Therefore, the correct answers are:
The first evening = 20 calls
The second evening = 30 calls
The third evening = 80 calls.