Question 1173046
.
<pre>

Every time, the inspector adds 2 good smartphones to 2 defective.


He can select 2 good smartphones from the 18 good smartphones in  {{{C[18]^2}}} ways = {{{(18*17)/2}}} = 9*17 = 153 ways.


It is the number of all possible favorable quadruples.


The number of all possible quadruples is, meanwhile,  {{{C[20]^4}}} = {{{(20*19*18*17)/(1*2*3*4)}}} = 4845.


THEREFORE, the probability under the problem's question is


    P = {{{favorable/total}}} = {{{C[18]^2/C[20]^4}}} = {{{153/4845}}} = {{{3/95}}} = 0.03157895.    <U>ANSWER</U>
</pre>

Solved.