a) 62.5 x 10³ is not in scientific notation because the number to the left of the decimal place needs to be between 1 and 9 (62 is not between 1 and 9). It should have been written as 6.25 x 10⁴
b) 6.57 x 1000 is not in scientific notation because the 1000 should be written as a power of 10. It should have been written as 6.57 x 10³
c) 0.39 x 10⁹ is not in scientific notation for the same reason as problem "a" above. The number to the left of the decimal place needs to be between 1 and 9 (0 is not between 1 and 9). It should have been written as 3.9 x 10⁸