We have a total of 13 3-year-olds, 20 4-year-olds, 15 5-year-olds, and x six-year-olds.
The number of students in the first class (3 and 4-year-olds) is 13 + 20 = 33.
The number of students in the second class (5 and 6-year-olds) is 15 + x.
The average class size is the total number of students divided by the number of classes, which is (33 + 15 + x) / 2.
We are given that the average class size is 35, so we can write: (33 + 15 + x) / 2 = 35.
Multiplying both sides by 2, we get: 33 + 15 + x = 70.
Simplifying the left side, we get: 48 + x = 70.
Subtracting 48 from both sides, we get: x = 22.
The value of x is 22.
#### 22
The answer is: 22
Below is a math problem. Provide a detailed, step-by-step solution.

### Problem:
There are 13 3-year-olds, 20 4-year-olds, 15 5-year-olds, and x six-year-olds at a particular Sunday school. If the 3 and 4-year-olds are in one class and the 5 and 6-year-olds are in another class, what is the average class size?
If we know the answer to the above question is 35, what is the value of unknown variable x?

### Answer:
