You are over thinking this, particularly if it doesn't need to be exact (although it would help if you told us which part did not need to be exact, the concentration or the volume)
If there are 0.75mg in 1ml, how many mg are there in 10ml, 100ml or 1000ml?
Still having trouble, if there is 30g per 1 bag of crisps, how many g in 10 bags?
I have a stock solution of powder (150g).
Do you mean you have 150g of powder or is there a volume missing here for your "stock solution"