Science math... I don't know where else to go. >_>

Yeah...Not sure if this qualifies for math help, but I don't know where else to go for this kind of help. Basically, I'm having trouble with Concentration Problems.

So I know to get the concentration% you have to do Solute/Solvent *100. But...some other problems are giving me trouble.

Basically I'm having trouble with the "How many grams of solute are in a solution with the following variables?" problems, and a few others. I don't get how to do it. I've even seen a couple worked by the teacher, but she's horrible at explaining mathematical equations. She pretty much worked the problem for the Concentration% and then wrote the gram answer on the board. Now, it says to *show your work*, and there's more than the two she worked out, so that's not gonna fly. Can someone tell me how to do these? I just need the method, and I can probably figure it out after that.

Some examples:

How many grams of solute are in a 20mL of a solution that contains 32g per 75mL?

A solution has 62.2g of barium chloride dissolved in 300mL of solution. How many grams of the solute are in 75mL of the solution?

How many milliliters of solution could be made with 130.47g of solute if it has a solubility of 87.32g?

How many grams of solute are needed to make 500 grams of a 7% solution?