55.

What do you mean by significant figures?
Or
What do you know about significant figures? What are the rules for determing the number of significant figures?

Significant figures. The total number of digits (including the last one, though its value is doubtful) in the number used to express the physical quantity is called the number of significant figures. For example, the reading 24 · 84 cm having three certain digits (2, 4 and 8) and one doubtful digit (4) has four significant figures.
Rules for determining the number of significant figures:
1. Except zero, all the digits 1, 2, 3, 4, 5, 6, 7, 8 and 9 are significant. For example,
4·25 has three significant figures.
71·435 has five significant figures.

2. All digits are significant including zeros if zero appears in between non-zero digits, e.g.,
4·02 has three significant figures.
4·005 has four significant figures.
6·0042 has five significant figures.

3. Zeros to the left of the first non-zero digit in a number are not significant, they merely indicate the positions of the decimal point. e.g.,
0·428 has three significant figures.
0·054 has two significant figures.
0·0006 has one significant figure.

4. Zeros to the right of the decimal point are significant. e.g.,
42·0 has three significant figures.
42·00 has four significant figures.
42·000 has five significant figures.

5. In exponential notation, the numerical portion gives the number of significant figures. e.g.,
1·58 × 10^–1 has three significant figures.
4·243 × 10⁵ has four significant figures.
3·24 × 10⁷ has three significant figures.