Science purpose for sci. fair?

ok so me and my friend are doing a science fair project where we are doing dance classes and r taking our heart rate before after and 15 mins. after each dance class. my purpose is -

Julia will take her heart rate to determine which dance class you will get the best cardio workout from

i have to make this purpose at least 3 sentences - i ask you to help

PLEASEEEE

Science purpose for sci. fair?

Julia will take her heart her heart rate before each dance class. Then, she will mark her heart rate after each dance class. with these results she can determine which will give you the best cardio work-out.