By the general expression for feedback; Af=A/(1+AF)

where (1+AF) is the feedback depth, A is the open-loop amplification and Af is the closed-loop amplification.

Known: open-loop voltage amplifier A = uo / ui = 2V / 2mV = 1000,

closed-loop voltage amplifier Af = uof / ui = 2V / 20mV = 100

In the general expression of the substitution of the given data:

100=1000 / (1 + AF)

Solutions of the depth of feedback (1+AF)=1000/100=10

Expressed in decibels: 20lg10=20db