Let x be the amount of calls that Carmen received at the third evening.
The first evening she received 6 calls fewer than the third, so that is x-6. The second evening, she received 3 times the number of calls of the third evening. So that is 3*x. The sum of this three quantities should be 74,sinche she received 74 calls in total. So, we have the equation
If we add all x values we get
If we add 6 on both sides, we get
Now, if we divide by 5 on both sides, we get
So in the third evening she received 16 calls. The first evening she received 16-6 = 10 calls and the second evening she received 16*3 = 48 calls.