ah so you're stuck on the basic math of chem
if you're having trouble there you should try org chem, that shit will kill you.
anyways, multiplication and division in chem is mostly the cancellations of labels. in doing so, I'd suggest using a yellow pad or a lined paper.
for the first one, I'm guessing that's a solution with a density of about 7.99 g/ml, then you have 25000ml of that solution and you want to get the mass of the solution. since one ml is above the division bar and one is under they cancel out so you're left with g.
pretty much just write out equations long on two rows such as this scan so you can easily cancel out labels.
for the last, remember diving is the same as multiplying the reciprocal (am I right?) of the number. so dividing by 4 is the same as multiplying by 1/4
Also, don't forget significant figures!! in the scan, the number of ? is the number of SFs you should use.