I think I had written it earlier, but here we go again, the number 20:20:20 or 5:10:15 or 0:20:50 etc denote the percentage of the elements Nitrogen:Phosphorus:Potassium (NPK) by weight in a fertilizer,
so 100g of 20-20-20 contains 20 g each of N,P,K to make a total of 60 g the remaining 40 g are contributed by other elements like Oxygen (O), Carbon (C), hydrogen (H), and may be Calcium (Ca), Magnesium (Mg) etc depending on the type of the fertilizer.
Now 1 g= 1000 mg
so 1 g 20-20-20 fertilizer contains 200 mg each of NPK
PPM (Parts Per Million) for calculation purposes is mg/litre or mg/Kg
1 kg = 1000g = 1000,000 mg
So if you add 1mg of Nitrogen to 1 litre of water you have a 1ppm solution (1 part of Nitrogen per 1 million parts of water)
Nitrogen fertilization is recommended between 100-300 ppm for our calculation purpose let us consider 200 ppm.
1 g of 20-20-20 fertilizer contains 200 mg Nitrogen, so when you add 1g of this fertilizer into 1litre (1000 g) of water, you have solution that has 200 ppm N, 200 ppm P and 200 ppm K.
If you want 100 ppm Nitrogen you dissolve 0.5 g fertilizer in 1 litre of water and if you want 300 ppm you dissolve 1.5 g fertilizer in a litre of water.
I hope this was helpful, feel free to ask if you have any more doubts.