Answer:
David find <u>8</u> quarters.
Step-by-step explanation:
Given:
David was in need for some money so he decided to clean out the water fountain at the mall he found 20 nickels and quarters.
The collection of nickels and quarters totaled to 2.6.
Now, to find the quarters David find.
Let the number of nickels be 
And the number of quarters be 
Thus, the number of nickels and quarters he found:

⇒
.........( 1 )
The totaled of nickels and quarters:
(As 1 nickel=$0.05 and 1 quarter=$0.25)
Now, putting the equation ( 1 ) in the place of
we get:

⇒ 
⇒ 
<em>Subtracting both sides by 1 we get:</em>
⇒ 
<em>Dividing both sides by 0.20 we get:</em>
⇒ 
So, the quarters = 8.
Therefore, David find 8 quarters.