what mass of AgBr is produced when 100.0 mL of 0.150 M AgNO3 is added to 20.0 mL of 1.00 M NaBr?

Answer 1

This is a limiting reagent (LR) problem since amounts are given for BOTH reactants.
AgNO3 + NaBr ==> AgBar + NaNO3

mols AgNO3 = M x L = ?
mols NaBr = M x L = ?

Convert mols AgNO3 to mols AgBr
Convert mols NaBr to mols AgBr.
It is likely the two values will not agree which means one of them is wrong; the correct value in LR problems is ALWAYS the smaller value and the reagent responsible for producing that number is the LR.
Now use the smaller value and convert to grams. grams AgBr = mols AgBr x molar mass AgBr.