How do I make 240m have only one sig fig?

Significant digits are used to limit a number to express
its true level of accuracy. The number 240 has two significant digits, the two and the
four. The zero is not considered significant in this instance because trailing zeros are
ignored unless a decimal point is present.

To get 240 down
to one significant digit, we need to round it off. Since the second digit is a 4, we
truncate, and our answer is 200. (Important note -  If
the second digit was a 5 or greater, we would round up and our
answer would be 300.)

An easy way to remember how to count
sig figs is the Atlantic/Pacific rule. Picture a map of the US, with an ocean on each
side. Then look at your number. If the decimal point is
Present, only ignore zeros on the
Pacific (left, or leading) side of the number. If the
decimal point is Absent, only ignore zeros on the
Atlantic (right, or trailing) side of the
number.