Answer
35%
Work Step by Step
Jill drinks 100mL so we will subtract this amount from 750mL (original amount), that is 650mL of 50% juice, she then refills the bottle (up to former 750mL) with 100mL of lower concentration (x) juice, the result is
48% pure juice. We will now solve for x:
0.5(650)+x(100) = 0.48(750)
325+100x=360
100x=35
x=0.35=35%