Question 1:
Answer: The number does not change
Removing the zero from 0.76 leads to .76 which is the same as before. Both lead to [tex]7.6 \times 10^{-1}[/tex] in scientific notation.
Personally I prefer the zero up front to make the decimal point stand out easier. Of course it's just a preference, so its up to you to decide which you prefer.
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Question 2:
Answer: Yes the number changes
Taking the zero out of 10.25 leads to 1.25 which is definitely a different number (and smaller). Therefore, this zero is a significant digit compared to before.
