A shop sells sweets in bags of 7 and 20 what is the largest number of sweets that cannot be purchased exactly?

By the Frobenius Coin Theorem, also known as the Postage Stamp Theorem or the Chicken McNugget Theorem, the maximum amount that cannot be represented as a sum of multiples of two relatively prime numbers a and b (numbers that have GCF of 1) is ab-a-b. Plugging 7 and 20 in, our answer is 7*20-7-20, or 140-27, or 113.