Thieves

Every evening, hundreds of students gather up at canteen and buy various snacks. Many of these students prefer to pay through E-Wallets and UPI methods such as PayTM, PhonePe and Google Pay etc. Due to the rush, the owner can't keep track of all payments and verify if a student has actually made the payment or not. But the canteen owner has visual memory i.e. he can remember a few digits of ph