Significant digits question

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240.00 is a 5 sig. fig. number
240 is a 2 sig. fig. number.
240. is a 3

That's correct. There's also another notation they can use besides a decimal, which is to put a line above the zero to indicate where significance ends. So basically, in Splat's last example, there could be a line above the zero instead of the decimal which means the same thing.