The number 725000000 when expressed in scientific notation,equal?

You might as well memorize this, because it is closely related:
physics.nist.gov/cuu/Units/prefixes.html

You just have to memorize this stuff, at least the ones you work with all the time. It's ok to bookmark the page so you can look up the others when you run across them.

There is a prefix every three powers of ten, except for 100, 10, 1/10, and 1/100. To go from one to another you use the LARD rule: LEFT ADD RIGHT DEDUCT. When moving the decimal to the LEFT you ADD to the power of ten. When going to the RIGHT you DEDUCT from the power of ten.

You have 725 million, so the prefix is mega and the scientific notation is 7.25 x 10^8. You move the decimal to the right of the first non-zero digit. Count the spaces you moved it, remembering the LARD rule, and that is the power of ten.