To the original question: Since "mg" means mg/ml that means a 10ml bottle of 18mg/ml contains 180mg of nicotine. (10ml * 18mg/ml, the "ml" cancels out, =180mg)
So using 24mg/ml to get to 180mg is 180mg/24mg/ml=7.5ml. (The "mg" cancels out.)
The remaining liquid is VG. 10ml-7.5ml=2.5ml of plain VG.
Result is a 10ml bottle of 18mg/ml juice.