How many grams of ferrous ammonium sulphate must be dissolved in 200ml water so that the solution is

physics.nist.gov/cuu/Units/prefixes.html

You just have to memorize this stuff, at least the ones you work with all the time. It's ok to bookmark the page so you can look up the others when you run across them.

There is a prefix every three powers of ten, except for 100, 10, 1/10, and 1/100. To go from one to another you use the LARD rule: LEFT ADD RIGHT DEDUCT. When moving the decimal to the LEFT you ADD to the power of ten. When going to the RIGHT you DEDUCT from the power of ten.

So 1 ppm means 1 mg Fe per liter. You are given 0.2 liter so you want 0.2 mg Fe. The formula for that is (NH4)2Fe(SO4)2·6H2O so all you have to do is look up the atomic mass of Fe (55.845) and that tells you that you need 0.2/55.845 = 3.58 x 10^-4 mole of the salt.

You do have a printed periodic table, right? You can get the information at www.chemicalelements.com/index.html but you really need your own printed copy to carry with you. It will be a primary reference for everything you do.