Let's set variables for water and vinegar:
water=w
vinegar=v
We can say that the volume of v plus the volume of w is 7.5 liters
v+w=7.5 liters
We also know that 60% of the original solution is vinegar. We can calculate how many liters of vinegar are in the original solution as follows:
.60 x 7.5 liters = 4.5 liters of vinegar. We can then subtract 4.5 from 7.5 to get the total volume of water in the original solution which would be 3 liters
So in our initial solution we have:
4.5 liters of vineger + 3 liters of water = 7.5 liters of solution.
If we add 1.5 liters of water we would get
4.5 liters of vineger + 4.5 liters of water = 9.0 liters of solution.
or half water and half vinegar. We can easily see that the percentage of vinegar in the new solution is 50%