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Mental Math

- Chapter 2
- Lesson 5

Let's Play

Solve

that

math

problem!

Oh wait

- There are some important math tools that you need

to learn before we can play.

1) Tips

- Mental math is always easier if you can use

numbers that end in - fives
- zeros

2) Compatible Numbers

- Pairs of numbers that are easy to compute
- They just go together!
- Example
- 25 18 75
- Add 25 75 first its easier
- Then add 18
- 100 18 118

3) Patterns

- Use when multiplying and dividing with zeros.
- Multiplying
- Multiply the numbers that are not zeros
- Add the number of zeros that are in the problem
- 20 x 700
- 2 x 7 14
- 1 zero 2 zeros 3 zeros (add these to the 14)
- 14, 000
- 6,000 x 400
- 2,400,000

Patterns cont

- Dividing
- Divide the numbers that are not zeros
- Subtract the number of zeros in the divisor from

the dividend - 540,000 900
- 54 9 6
- 4 zeros 2 zeros 2 zeros (put the with the 54)
- 600
- 21,000 3,000
- 7

4) Compensation

- Compensation is choosing an easier number to use

and then adjusting the final answer. - 58 x 3
- 60 x 3 is easier
- 180
- Butwe have to take away what we added
- 2 x 3 6
- 180 6 174
- 42 x 8

Distributive Property

- Break big numbers into smaller, simpler problems
- 32 x 5
- (30 x 5) (2 x 5)
- 150 10 160
- 56 x 8
- (50 x 8) (6 x 8)
- 400 48 448

Now...

Let's Play

Solve

that

math

problem!

Directions

- 2 players from each team will meet at the front

of the room - They will shake hands
- The teacher will tell the competitors the type of

problem they will be given (Multiplication,

Division, Addition, Subtraction) - The players will then start the bidding

Directions cont

- The first player will state how many seconds

he/she can answer the problem (35 seconds). - The other student can counter by stating they can

answer the problem in a shorter amount of time

(30 seconds) or the student can tell the other

opponent to Solve that Math Problem! - The students can go back and forth until one of

the competitors say Solve that Math Problem!

Directions cont

- The teacher will then give the actual problem.
- The problem must be solved mentally
- If the student answers correctly in the

pre-determined amount of time, he/she gets the

point. - If incorrect, the opponent gets the point