Each change of two rear tires changes a front. It's also very annoying chat. Do the math: if the rear tire is changed with 15,000 km, the front tire will have the same 15,000 km, after all, they go together. Then the guy puts on a brand new rear tire and leaves the front used. When this bike runs more than 5,000 km you will have a new rear tire, tractioning well and a front tire with 20,000 km without conditions to roll. Stop being crusty and always change both!