- (5 min) Mental Math
- What is the probability of flipping a coin 3 times

and getting a 'heads' all 3 times? [1/8] - Start with 2400, divide by 100, then divide by 3 [8]
- What is the probability of drawing a red 6

out of a deck of 52 cards? [1/26] - What is 20% of 80? [16]
- Multiply 1/4 by 4/5 and reduce [1/5]

- What is the probability of flipping a coin 3 times
- (5 min) Review of homework #22 (<= 3 problems)

- (10 min) Discuss adding and subtracting like factors:

- Use this example to introduce adding X's:

X + 2X = 27

Point out that 2X means that there are 2 X's there. So

X + 2X = 3X, so

3X = 27,

Therefore X = 9, because 3 X 9 = 27. - Key point: So 3X means 2 things:
- 3 X's
- 3 times X

- Use a subtraction example:

4X - 2X = 18 - Applications in geometry:
- Draw a rectangle twice as wide as it is high and label 2 of
the sides "X" and "2X". Construct the equation for the perimeter:

X + 2X + X + 2X = 24 inches.

What is the length of the short side?

6X = 24

X = 4. - Draw a tall isosceles triangle with angles X, 2X, and
2X.

Construct an equation for the sum of the angles:

X + 2X + 2X = 180

What is the smallest angle?

5X = 180

X = 36 degrees!

- Draw a rectangle twice as wide as it is high and label 2 of
the sides "X" and "2X". Construct the equation for the perimeter:

- Use this example to introduce adding X's:
- (Remainder of class) In-class Exercise.

- Hand out homework as students successfully complete the in-class exercise.

Note: Biggest student problem with this lesson:

Understanding that for the equation:

2X + X = 9,

when you ask "How many X's are there? that the answer is not 2. This means understanding that 2X really means that there are 2 X's here.