[Lackydoo]

You are asked to prepare a 15% solution of H2SO4 from 1.5 kg of the 20% solution. What amount of water should you add?

20% = 300g H2SO4
80% = 1200g H2O

In the new solution the amount of H2SO4 should still be the same right? That means 300g H2SO4 should be 15%.
300g/15 = 20g/1%
85*20=1700.
So the answer must be to add 200g of water?

Is this correct or am I doing something wrong?

[AdiDex]

No you are doing something wrong , if you add 200 water that means there is 300 g H2SO4 is in 1700 g of  solution , that means your solution is 300/1700 *100% = 17.647%(w/w) .

you have to dilute it further to get required composition .

[Borek]

In the new solution the amount of H2SO4 should still be the same right? That means 300g H2SO4 should be 15%.

Yes, that's correct.

300g/15 = 20g/1%
85*20=1700.

But this doesn't make much sense (or rather: the sense it makes is not the one you think it is).

What is the mass of the solution of which 15 % is 300g?

[Lackydoo]

In the new solution the amount of H2SO4 should still be the same right? That means 300g H2SO4 should be 15%.

Yes, that's correct.

300g/15 = 20g/1%
85*20=1700.

But this doesn't make much sense (or rather: the sense it makes is not the one you think it is).

What is the mass of the solution of which 15 % is 300g?

Yes you are correct. It doesn't make any sense now that I know the answer. The mass has to be 2000. Therefor we add 500g of water.

Thank you for the help, both of you!