McDonalds is one of the most popular fast food brands in the world, with around 36,000 branches worldwide and more than 1200 in the UK alone. The affordability of their meals is one of the company’s major selling points. But now you can reduce the cost of a McDonalds staple - Big Mac and fries - by more than half, using a simple trick. In order to take advantage of the offer, you’ll need a receipt issued in the last 60 days from McDonalds for any purchase.The offer is only open to people aged 16 and over.At the bottom of every receipt from McDonalds there is a unique 12-digit code.If you enter the code on the fast food firm’s survey website, McDonalds Food for Thoughts, you will then be asked to fill in a quick survey.You’ll need to share some thoughts on the food you purchased, but once you’ve finished the quick survey you’ll receive a five-digit code.You then need to write down the five-digit code on the receipt, which will then work as a voucher at any McDonalds store.The voucher will allow you to get a Big Mac, Fillet-O-Fish or Vegetable Deluxe, with either fries or a side salad, for the discounted cost of £1.99.The cost of a Big Mac and fries is typically £4.79, meaning a saving of £2.80.The trick can be used as many times as you like, but you’ll need a different receipt each time.The unique 12-digit codes are printed on every receipt, regardless of the amount of purchase.This means that in theory, you could purchase a low-value item to receive a receipt with a code, which you can then use to get the discounted burger and fries.