Final answer:
The number 6.50 has three significant digits because it includes one trailing zero after the decimal, which is considered significant.
Step-by-step explanation:
The number 6.50 has three significant digits. When determining significant figures, we need to consider several rules. Non-zero digits are always significant, any zeros between non-zero digits are significant, and trailing zeros in a decimal number are also significant.
In the case of 6.50, the '6' is a non-zero number and thus significant, the '5' is a non-zero and thus significant, and the trailing zero, because it is after the decimal point, is significant as well. Therefore, the number 6.50 has three significant figures.