Hopefully you can zoom into the image.
1. Zeros at the beginning of a number are never significant.
2. Zeros at the end are only significant if there is a decimal place. 300 (1SF), 300. (3SF), 300.0 (4SF)
3. Zeros in the middle are always significant.
When adding or subtracting, the answer can have no more decimal places than the measurement with the fewest number of decimal places.
When multiplying or dividing, the answer must be given with the same number of significant figures as the measurement with the fewest significant figures.