Answer: Let's assume the scientist uses x ounces of solution A and y ounces of solution B to obtain 90 ounces of the mixture with 75% salt.
We need to set up a system of equations based on the given information:
The total amount of mixture: x + y = 90 ounces
The total amount of salt in the mixture: 0.65x + 0.80y = 0.75 * 90
Now, let's solve this system of equations to find the values of x and y:
From the first equation, we can express y in terms of x:
y = 90 - x
Now, substitute this value of y in the second equation:
0.65x + 0.80(90 - x) = 67.5
Now, solve for x:
0.65x + 72 - 0.80x = 67.5
-0.15x = -4.5
x = 30
Now that we have the value of x, we can find y:
y = 90 - x
y = 90 - 30
y = 60
So, the scientist should use 30 ounces of solution A and 60 ounces of solution B to obtain 90 ounces of the mixture with 75% salt.