Quick questions on Scientific notation and significant figures for HSC Maths Standard 2

For a number bigger than $10$, put the decimal point after the first non-zero digit and count the places back to where it started (the right-hand end). The Earth-Sun distance is

For a number between $0$ and $1$, the point moves the other way, to the right, until it sits just after the first non-zero digit, and the power is negative. An atom's radius of

A calculator shows standard form to save space. A display like 6.022E23 or 6.022^23 means $6.022 \times 10^{23}$, not $6.022^{23}$. The E (or a small raised number) stands for "times ten to the power". A display of 4.5E-7 means $4.5 \times 10^{-7}$.

Significant figures are the digits in a number that carry real information about its size and accuracy, as opposed to zeros that are only holding place value. The rules for which digits count are:

To round to $n$ significant figures, keep the first $n$ significant digits, then look at the next digit: if it is $5$ or more, round the last kept digit up; otherwise leave it. The place-holding zeros (or the power of $10$) are then set to keep the number the right size. For example,

Large numbers take a positive power, numbers below $1$ take a negative power. A negative power does not make the number negative; $2.5 \times 10^{-10}$ is a small positive number.