How many liters of 1.00 M HCl must be added to 50.0 ml of 0.500 M HCl to give a solution whose

concentration is 0.750 M?

moles = M x L

moles soln1 + moles soln2 = moles soln 3
(0.500M*0.0500L) + (1.00M*xL) = 0.750M*(0.0500+xL)
Solve for xL.
If I didn't make a math error the answer is 50 mL.