Digital design, which system(binary,octal or whatsoever) are they using?

a. You meet an alien! He waves at you with his four hands... and gives you 29 apples as a sigrn of his friendship. However, he says he gave you 19, not 29 apples!! (But you are both right.) How many fingers does he have on each hand? b. He says he will be back in 100 (Earth) years! What (Earth) year will that be? It is 2018 now