At the beginning there was just 1 mole of hydrogen and 1 mole of iodin.
The reaction goes:
[tex]H_2+I_2\Rightarrow\ 2HI[/tex]
One mole of hydrogen goes with one mole of iodin, creating 2 moles of product.
If 0.8 of hydrogen remaind, it means that 1-0.8=0.2 reacted.
So:
1 mole H2 react with 1 mole I2 creating 2 moles of products.
So 0.2 of hydrogen reacted with 0.2 mole of iodin creating 0.4 moles.
[tex]1\ mole\ H_2\ ---\ 2\ moles\ HI\\
0.2\ mole\ \ \ ---\ x\\
x=0.4[/tex]