Lucas is the owner of a shoe shop. One day, a customer bought a pair of shoes costing £$30$ with a £$50$ note. Lucas then went to the supermarket for some change. Afterwards, the owner of the supermarket found out that the £$50$ note was fake, and Lucas had to pay the £$50$ back. Knowing that the shoes cost £$30$, how much money did Lucas lose that day? 
Overseas Competition->Knowledge Point->Word Problem Modules->Applying Addition and Subtraction->Simple Addition and Subtraction