Step 1 We identify the starting point in the equation.

This is always the point that we can calculate an amount of substance. In this case we know we have 5 moles of oxygen gas.

Step 2 Identify the finish.
This is always the subject we need to find out. In this case it is the amount of water in mole

Step 3 Using the coefficients in the balanced equation derive the ratio. Which is always finish divided by start.
2/1 = 2

Step 4 Multiply the ratio by the number of moles given at the start.

ratio X start = moles of water produced.

2X 5 moles of hydrogen = 10 moles of water