Question 999695
The first question is a dilution.  You are adding a material which contains zero amount of the solute to be diluted.

{{{5*5/(v+5)=4}}}



Second question is two-part mixture for known amount of resulting mixture.  You can do all in one single variable if you want.
y, the gallons volume of the 10%
10-y, the volume of the 40%
-
{{{(10y+(10-y)40)/10=20}}}



Check the available lessons.
<a href="http://www.algebra.com/tutors/aboutme.mpl?userid=josgarithmetic">Available lessons, some of them being for Mixture problems</a>