Answer
$35\%$
Work Step by Step
The amount of pure fruit juice in the original bottle is $50\%$ of 750mL, or 375mL. Jill drank 100mL of the punch, meaning she drank 50mL of pure fruit juice. Therefore, there is 325mL of punch left in the bottle now. When she refills the bottle, the concentration is $48\%$ juice, meaning there is 360mL of juice now. This means that the increase in juice is $360 - 325 = 35mL$. Since she added 100mL to the bottle, and 35mL of that was juice, the concentration of the new punch is $\frac{35}{100} = 35\%$ juice.
