Question 1118213
<br>
The strength of the desired solution is 1/20 of the strength of the stock solution (5 mg/mL compared to 100 mg/mL).  That means 1/20 of the mixture must be the stock solution.<br>
1/20 of 20 mL is 1 mL.<br>
You need 1 mL of the stock solution and 1 mL of the diluent.<br>
--------------------------<br>
Sorry.  That last line was a typo.  The work I show leading up to my statement of the answer clearly shows that the "1mL" of diluent was supposed to be 19mL.