Tell them to pick a number any number, but not to tell you.

Then ask them for example to add 5 to there number, then add 12. Then add 8,then add 5. Then subtract 10 Then subtract 2. Then subtract 8.

Then you ask them to subtract the number they started with, there secret number( this number is of no importance as you will see)
Now that they subtracted there number they should have 10.
It does not matter what you tell them to add and subtract as long as your keeping track. Once they subtracted their secret number you can start to multiply and divide.

where were we (at 10) now ask them to multiply by 10. Now ask them to divide by 5. ask them to add 1 and then ask them to divide by 3.

Know look at them as if reading there mind, and say something like" the number your thinking of is 7."

Remember add and subtract first, then ask them to subtract the number they started with(their secret number). Then multiply and divide.
keep track of what your adding, subtracting, multiplying, and dividing. Them having to do the math will keep them from thinking what the trick is.
I had fun with this trick, good luck.
P.S. this trick works well one on one, I don’t know about groups. Although you could do it to an entire class no matter what secret number they pick, following the example above they would all end up with 7.