You use the mole ratio that exists between "A"A and "B"B.
Let's say your balanced equation looks like this
"A" + color(red)(2)"B" -> "products"A+2B→products
If you have 1 mole of "A"A, you'd need color(red)(2)2 moles of "B"B for the reaction to take place. This means that you have a 1:21:2 mole ratio between "A"A and "B"B.
So, if you have xx moles of "A"A, you'd have
xcancel("molesA") * (color(red)(2)" moles B")/(1cancel("mole A")) = "2x moles B"
So, if you know the number of moles of a species involved in the reaction, you can automatically use the stoichiometric coefficients to determine the number of moles of the rest of the species present.
If you have a balanced chemical equation that looks like this
color(red)(2)"A" + C -> color(blue)(3)"B" + D
You'd have a color(red)(2):color(blue)(3) mole ratio between "A" and "B", so you can go from moles of "A" to moles of "B" by multiplying by 3/2
"x"cancel("moles A") * (color(blue)(3)" moles B")/(color(red)(2)cancel("moles A")) = 3/2"x moles B"
