I can just manage to do 20 in one sitting but would it matter if i kept doing 10 then rest 10 then rest again?I can just manage 25 and my arms are too tired.
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From personal experience, it's better to do as many as you can instead of breaking up that number into more sets. That being said, 20-30 isn't too many and if you keep it up you'll go beyond that number pretty quickly.
Google "grease the groove"If you can manage 20, then do 17-18 pushups in one sitting. Throughout the day keep doing sets like that.
If you have to, go to your knees and keep doing the pushups.