The order is given for an injection of 120mg of x and 10mg of y. On the shelf you have 50mg/mL of x and 75mg/mL of y. what will be the total volume in your syringe?
50 mg/mL x ?mL = 120 mg
75 mg/mL x ?mL = 10 mg
Solve for mL in each case, then,
add ?mL and ?mL = total mL.