M or molar is a unit of concentration. Molar is equivalent to the number of moles of a solute over the volume of the solution. If the volume of that solution is already given, the number of moles of solute can be calculated. In this problem, the principle we have to put in mind is that in each of the solutions, the moles of HCl must be constant. So the volume must be the only thing changing. The equation will then be:
V1(M1) = V2(M2)
V1(3.0M) = 0.250L(1.2M)
answer is 0.1L or 100ml.
