Tip-calculator

In this projected i used html and css with javascript But i had an issue and till now I didn't get a solution :( The problem is that, when I run my HTML file and input the amount in the input section and try and click on any percentage it those not work, but when I refreshed the page and continue with the amount I put in the input, it then works but if i delete the amount I had put and put a new one the input field is not update and if i press any percentage, it still work with the previous amount that was inside even if the value has change :(. I do really need help on this.. THANKS