To know the number of significant values, we can use the following rules:
1. Every number that is different from 0 is significant
2. If the zero is between two digits, it is significant.
3. the zeros on the left of the first significant figure are not significant
4. the zeros on the right of the first significant figure are significant
So, in this case:
7.8 has 2 significant figures
54 has 2 significant figures
0.0074 has 2 significant figures
6400.7 has 5 significant value
Answers: 7.8, 54, and 0.0074