Tricky Contracts

Main to abhi tak kuch samajh gaya hua hai, lekin is question ke baad kyun hua hai. Isme likha tha, "Ankit hired a bike from Bikram for a period of one month on a monthly rent of Rs. 500. Ankit, however, returned the bike after 20 days. Bikram sent Ankit a notice demanding immediate return of the bike. Ankit claimed that the contract was not concluded because of mutual mistake of fact. Can Bikram sue Ankit for not returning the bike?" Main sochta tha ki Ankit ka koi right hai, lekin actual answer theek nahi tha. Correct approach yeh hai ki Ankit aur Bikram ne ek valid contract banaya tha. Ankit ka 20 days ke baad bike vapas karna thoda galat tha, aur Bikram ka notice bhi theek tha.