If you want 24mg nic juice, you will need 15ml of 48mg nic.
Basically the math goes;
Ok, what percentage of 48mg is 24mg?
So your juice has to be 50% nic solution to give you a 24mg juice.
=50% of 30ml = 15ml.
If you started with 100mg nic, and you wanted to make 24mg, 24mg of 100mg is 24%
24% of 30ml = 7.5ml of nic to make a 24mg