The bottle has 100mg per 15ml...
We need 40 mg which is 2/5 of the 100 mg
You need to take 2/5 of the 15 ml to get 40 mg of medicine
(2/5)(15) = 6 = 6 ml
If you are using a 1 ml calibrated glass dropper you would have
to fill it 6 times... I would suggest the little cup.