4.what is the quotient written in scientific notation 3x10^20/4x10^5

The quotient is
7.5  × 10¹⁴


the  × still means multiplication. thus  × and  · mean the same operation

we have the following quotient

   

we can rewrite so that it becomes

   

3/4 = 0.75, which can be written as 7.5 × 10⁻¹ in scientific notation.
10²⁰ / 10⁵ is 10²⁰⁻⁵ = 10¹⁵ by exponent laws. then we have

   

if we multiply numbers with the same base, we add the exponents. thus the final answer is