2. Example:
Mark thinks of an number
He subtracts 3
His answer is 12
What was the number he thought of?
The unknown quantity is the number he started with.
Let that be n
Construct an expression: subtract 3 n-3
Form an equation: the answer is 12 n-3=12
Solve the equation: the opposite to - is + n=12+3
n+15
Check the answer: 15-3=12
Mark thought of 15!

3. Example
One angel of an isosceles triangle is 1100
What are the other two angles?
The unknown quantity is the size of the equal angles.
Let each angle be a
Construct an expression the angle sum: a+a+1100
Simplify the expression: 2a+1100
Form an equation: angle sum is a+1100=1800
Solve the equation: a=
2a=70
a=70/2
a= 350

4. Try this:
Sean thinks of a number, halves it and ands 8. The answer is 20. What was the number?