In scientific notation, the number 39,940,000,000 becomes a. 3.994 × 1010. b. 3.994 × 10–10. c. 39.94 × 109. d. 3994 × 107.

you need to move the decimal 10 places to the left to get it between the 3 and 9

 so it would be 3.994 x 10^10

hi,

3. 12.15, you are correct.

4.2.01, your first digit is correct,

hope this !