Answer :
Step 1: Define significant figure
The significant figures of a number are digits in the number that are reliable and absolutely necessary to indicate the quantity of something
Step 2: State the rules of significant figures applicable to the given numbers
Rule a: One of the rules of identifying significant figures in a number is that non-zero digits within the given measurement are significant e.g. 452 has 3 significant figures
Rule b: The zeros between two significant non-zero numbers are significant. e.g. 23.005 consists of 5 significant figures
Step 3: Apply the rules to get the number of significant figures of the question given
Applying rule a, the given number 7.405 has 7, 4, and 5 are nonzeros digits making 3 nonzeros.
Applying rule b, even 0 is a zero digit but it would be counted as a significant figure because it is between nonzeros numbers 4 and 5
Hence, the number of significant figures of 7.405 is 4