Suppose you had 2.0158 grams of hydrogen (h2).
how many moles of hydrogen do you have?
how many moles of oxygen would react with this much hydrogen?
what mass of oxygen would you need for this reaction?
how many grams of water would you produce?

A.How many moles of hydrogen do you have? 
2.0158 g H2 ( 1 mol / 2.02 g ) = 1.00 mol H2

B. How many moles of oxygen would react with this much hydrogen?The chemical reaction would be:
H2 + 1/2O2 = H2O

1.00 mol H2 ( 1/2 mol O2 / 1 mol H2 ) = 0.50 mol O2

C. What mass of oxygen would you need for this reaction? 
0.50 mol O2 (32 g / 1 mol ) = 16.00 g O2

D. How many grams of water would you produce?
1.00 mol H2 ( 1 mol H2O / 1 mol H2 ) ( 18.02 g H2O / 1 mol H2O ) = 18.02 g H2O

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