Answer: The answer is 22 pounds of Strawberries and 8 pounds of blueberries.
Step-by-step explanation: Let, Estelle should 'x' pounds of Strawberries and 'y' pounds of Blueberries in the fruit salad. Then according to the question, we have
[tex]x+y=30.[/tex]
Also, Strawberries cost $1.80per pound and Blueberries cost $4.50per pound, so we can write
[tex]1.80x+4.50y=30\times 2.52\\\\\Rightarrow \dfrac{9}{5}x+\dfrac{9}{2}y=\dfrac{3\times 126}{5}\\\\\Rightarrow \dfrac{1}{5}x+\dfrac{1}{2}y=\dfrac{42}{5}\\\\\Rightarrow \dfrac{2x+5y}{10}=\dfrac{42}{5}\\\\\Rightarrow 2x+5y=84.[/tex]
From the first equation, we get
[tex]2x+2y=60.[/tex]
Therefore, taking the difference of the above two equations, we have
[tex]5y-2y=84-60\\\\\Rightarrow 3y=24\\\\\Rightarrow y=8,[/tex]
and
[tex]x=30-8=22.[/tex]
Thus, Estelle should use 22 pounds of Strawberries and 8 pounds of Blueberries in the fruit salad.