We know that 10 children have 7 books each, so they have a total of 10 * 7 = 70 books.
The teacher brings another x books to the classroom, so the total number of books is 70 + x.
We are given that the total number of books is 78, so we can write: 70 + x = 78.
Solving for x, we get: x = 8.
The value of x is 8.
#### 8
The answer is: 8
Below is a math problem. Provide a detailed, step-by-step solution.

### Problem:
In a classroom, 10 children have 7 books each. Their teacher brings another x books to the classroom. How many books are in the classroom altogether?
If we know the answer to the above question is 78, what is the value of unknown variable x?

### Answer:
