Calculating fun

Here are a couple of the tricks we did last week.

Seven Up
Choose a number (up to 8 digits)
Double that number
Subtract 16 from that number
Multiply the answer by 4
Divide that number by 8
Add 15 to that answer
Subtract the original number

The answer will always come up as 7.
Anyone want to figure out why? It's not magic, it's math.

Pair of Dice
Roll a pair of dice to get two numbers (1-6) - or you can just choose them
Multiply the first number by 5
Add 12 to that answer
Double the total
Add the second number to the result
Add 15 to that answer
Subtract 39 and you will get the two numbers on the original dice

Double Trouble

Pick any 1-, 2-, or 3-digit number
Multiply by 7
Multiply that answer by 11
Multiply that answer by 13
You will get the original number doubled