multiplying and dividing in scientific notation

When dividing numbers in scientific notation, what do you do to the exponents?
For example:
(4×10⁵) ∕ (3×10²)

• Add them
• Subtract them
• Keep them the same
• Divide them

(2 x 10⁹)(4 x 10^-4)

• 8 x 10¹³
• 8 x 10³⁶
• 8 x 10⁵
• 8 x 10^-36

(9.6×10³) × (6.7×10²)

• 64.32×10⁵

• 6.432×10⁶

• 64.32×10⁶

• $64.32\times10^3$

(5x10¹⁰) x (3x10⁷)

• 15x10¹⁷
• 1.5x10¹⁷
• 1.5x10¹⁸
• 1.5x9¹⁷

Solve:
(2.1×10​²​​) × (3.0×10​⁴)

• 5.1 x 10²
• 6.3 x 10⁶
• 5.1 x 10⁶
• 6.3 x 10⁸

Solve:
(6 x 10⁶) / (2 x 10³)

• 12 x 10³
• 3 x 10⁹
• 1.2 x 10⁴
• 3 x 10³

Evaluate. Leave your answer in scientific notation. 
(9.6×10⁸)  ⁄  (3×10⁴)
(hint - it's division...)

• 6.6×10²
• 3.2×10¹²
• 3.2×10⁴
• 6.6×10⁴

Solve:

(20 x 10⁵⁰) / (4 x 10¹⁵)

• 2.4 x 10⁵¹

• 5 x 10⁶⁵

• 8 x 10⁶⁶

• 5 x 10³⁵

What is the simplified version of

• 3.9 x 10⁶
• 4 x 10²⁰
• 4 x 10⁶
• 3.9 x 10²⁰

(3 x 10^-5)/(4 x 10²)

• .75 x 10^-3
• .75 x 10^-7
• 7.5 x 10^-8
•  7 x 10²