At the beginning there was just 1mole of hydrogen and 1 mole of iodin. The reaction goes:
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.