Answer:
Part a:
Linear equation: 1.25x + 0.50y = 2.75
Part b: 0.8 pounds of apples
Explanation:
Part a.
Let x represent the number of pounds of apples that Diego bought and let y represent the number of pounds of bananas that he bought.
Since, bananas cost $0.50 per pound and apples cost
$1.25 per pound.
Diego purchased bananas and apples and spent $2.75.
The linear equation in standard form for Diego's purchase will be:

Part b.
Since Diego purchased 3.5 pounds of bananas, we can substitute y = 3.5 into the equation to find the number of pounds of apples he bought:
0.5(3.5) + 1.25x = 2.75
1.75 + 1.25x = 2.75
Subtract 1.75 on both sides:
1.75 + 1.25x - 1.75 = 2.75 - 1.75
1.25x = 1
Divide both sides by 1.25.

x = 0.8
Therefore, Diego bought 0.8 pounds of apples.