tell her you want to stay alive for a long time and that u really need exercise and u will do anything for her like give her a massage or make her food and stuff!
simple just tell your mum how fat you are looking n wear some lose fitted clothes n tell her your boyfriend said you he gonna leave you if you put on more weight and all after that she don't buy you an excersise bike just ask your dad
You have to really sell her the benifits. Or earn it, strike up a deal that she cant refuse where you do something for her in exchange for an exercise bike.
Tell her the benefits of you getting exercise and make a promise that you'll use it so it'll be worth the purchase! It'll benefit her daughter and help her lead a healthy lifestyle! Who doesn't want their children to be healthy and happy?!?!