Key Concepts:
Be careful with zeros. Zeros in between non-zero numbers are significant. Zeros after the decimal are significant ONLY IF they FOLLOW any OTHER number.

Examples:

a.) 4000000 - has 1 significant digit (the zeros are there as place values showing size of the number)

b.) 95.00 - has 4 significant digits (we DON'T need those zeros because without them the value remains the same, HOWEVER, by us including those zeros we MAKE THEM SIGNIFICANT)

c.) 0.00003 - has 1 significant digit (zeros in front of the three are only there as place values. Without them the value of the number changes)