When the solutions of HCl with different concentrations and different volumes are mixed, The resultant concentration of solution can be calculated in the following way..
The solution of this problem is based on the rule of dilution. A solution of molarity M1 and volume V1 is diluted to V2 and of molarity M2, then, the resultant molarity can be calculated using this dilution equation.
M1V1 = M2V2
100 mL of 1 M HCl, 200 mL of 2 M HCl and 300 mL of 3 M HCl are mixed. The molarity of the resulting solution is 1) 1 M 2) 2.66 M 3) 2.33 M 4) 4.25 M