Mental Math

1
Mental Math

• Chapter 2
• Lesson 5

2
Let's Play
Solve
that
math
problem!
3
Oh wait

• There are some important math tools that you need
  to learn before we can play.

4
1) Tips

• Mental math is always easier if you can use
  numbers that end in
• fives
• zeros

5
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

6
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

7
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

8
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

9
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

10
Now...
11
Let's Play
Solve
that
math
problem!
12
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

13
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!

14
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