How many H2 and O2 molecules do you need to make exactly 4 water molecules with no leftovers?

You need 4 H2 molecules and 2 O2 molecules.
This is because water is H2O which is 2 Hydrogen and 1 Oxygen so if you want to make 4 water molecules, you’ll need 8 hydrogen and 4 oxygen.
Given H2 you need 4 because 4•2 is 8 and for O2 you need 2 because 2•2 is 4.
I hope this makes sense.

the catch is that the top numbers are atomic masses the bottom numbers are the numbers of protons and electrons in an uncharged atom. they are the number of that atoms as you would find them on the periodic table. further, the numbers on the left are used to determine the missing numbers on the right

so here we go

atomic mass

239 + 1 = y + 134 + 3 combine like terms

240 = y + 137             subtract 137 from both terms

240 - 137 = y

103 = y

atomic number

94 = z +54 + 0

94 = z + 54               subtract 54 from both sides.

40 = z

answer