>French numbers (up to 100) are combination of the decimal system (based on 10), for numbers up to 60; and vigesimal system (based on 20) which they use for 70, 80, 90. So 80 = 20 x 4, 90 = (20 x 4) + 10. Some other french-speaking countries (like Belgium) use decimal system for all numbers; including 70, 80, 90.
>But this is an easier part of French language :)

not for 80 that's canadian french (octante).