To decrease £840 by 5%, you need to find 5% of 840. Once you find the 5% of 840, you then subtract that number by 840. To solve your question, you would do 840 divided by 100, which gives you 8.4. Then, to find the 5% (because the 8.4 is only 1%), you multiply 8.4 by 5, which gives you 42. Then, all you have to do is subtract 42 from 840, which leaves you with 798. The answer to your question then would be £798.
I hope I helped, and if you need more information or see that my answer is incorrect, please tell me so I can fix it! :D
