399+428 make a ten or hundred to add mentally

827

Step-by-step explanation:

Hi there!

For any addition and subtraction, if you add and subtract 1 (or any other number), the result remains the same. In this case

399 + 428 = 399 + 428 + 1 - 1

Reordering

(399 + 1) + (428 -1)

Which gives

400 + 427

Which can easily solve to get 827

0.3643

Step-by-step explanation:

Standard Form and Scientific Notation

What is standard form of numbers?

Standard form is the normal way of writing numbers no matter how big or small the number is.

Examples:

15,000,000

0.0000000678

What is scientific notation?

Scientific notation is a different way of expressing numbers. It is a short way of expressing very big numbers or very small numbers. It is also known as exponential form. Scientific notation is written as the product of a number and a power of 10.

Examples:

1.3 × 10⁶

34.5 × 10^⁻9

Going back to the question above, we need to convert the given scientific notation to its standard form.

Converting of Scientific Notation to Standard Form

To convert a number from scientific notation to standard form, move the decimal point.

The exponent tells the direction and the number of times we need to move the decimal point.

The positive exponent indicates a large number so we need to move the decimal point to the right.

The negative exponent indicates a small number so we need to move the decimal point to the left.

Do not write the power of 10 anymore.

Solution:

Now, let us convert 3.643 x 10^-1 in standard form.

The exponent is negative 1 so we will move the decimal point to the left once.

3.643 = 0.3643