Answer:
91 mL
Step-by-step explanation:
In chemistry, to solve this kind of exercises, dilutions, and concentrations is common to use the following equation
C1*V1=C2*V2
In which V1 refers to volume 1, in this case, is 70mL because is the quantity I have, the solution I can use to obtain another solution. C1 refers to the concentration of the solution I also have, in this case, 65%.
C2 and V2 refer to the volume and concentration of the solution I am looking for. So, if want to know V2, which is the volume I need to add, the only thing to do is find V2 in the equation:
V2=C1*V1/C2
V2= 65%*70mL/50%
V2= 91mL