mental math: Describe a way to mentally evaluate 6 – 1.95 w/ no pencil, paper, or...

[sdmf8707]

mental math: Describe a way to mentally evaluate 6 – 1.95 w/ no pencil, paper, or...

determine how you can find the value of each expression below without pencil, paper, or calculator. Usually, mental math does not involve traditional algorithms (rules and "steps") used for pencil/paper calculations.
a) Describe a way to mentally evaluate 6 – 1.95 with no pencil, paper, or calculator.
b) Describe a way to mentally evaluate 6 ÷ 0.2 with no pencil, paper, or calculator.

 

[mmm4444bot]

Addition and subtraction undo each other.

Multiplication and division undo each other.

Multiplying or dividing by tens shifts a decimal point to the right or left, respectfully.

We can use these facts to help us with mental arithmetic.

A simple example:

1.2 × 12

We can first multiply 1.2 by 10, to get 12 (we'll undo that multiplication with division, at the end). We already know that 12×12 is 144. Dividing 144 by 10 gives the answer: 14.4

The steps look like: 1.2 × 10 × 12 ÷ 10

Another example:

77 - 21.3

If we subtract an additional 0.7, we'll subtract a total of 22 (we'll undo that subtraction with addition, at the end). We can mentally subtract 22 from 77, to get 55. Adding 0.7 to 55 gives the answer: 55.7

The steps look like: 77 - 21.3 - 0.7 + 0.7

I hope these examples give you some ideas. :cool: