Short but fast run vs slow but long run?

I usually run on the treadmill for 10-20min at a full on run and take a few breaks (30 seconds) maybe 3 breaks. My sister does a walk/jog for about 30+ minutes. I am running to burn calories and get abs and get rid of body fat. So is my sister. which routine is more effective?

This sounds like a math problem! LOL
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