How do you use the guess and check method to solve mathematical sentences

The guess and check method is exactly as the name implies.
1. Take a number that you think would work. I tend to use multiples of five, such as 10, to start.
2. Take this random number and plug it into the equation.
3. Solve the equation using that number.
4. Check it. Ask yourself these questions: Is the answer I got equal to the total it's supposed to be equal to? Is it a bigger number, or is it smaller?
5. If it's bigger by a lot, try a much smaller number. If it's bigger by a little, try a number just a little less.
The same rule works in the case that the answer you got is smaller than what the equation should be equal to. If it's much smaller, use a much bigger number. If it's just a little too small, use a little bigger number.
6. Repeat this process until you've found the number that is the correct answer.

It's a time-consuming process, but once you've had practice, it gets easier to guess closer to the number right off the bat.
Good luck!