Many people calculate wrong

Explanation of the steps:

Cat ++ Mouse =8kg=8𝑘𝑔

Dog ++ Mouse =11kg=11𝑘𝑔

Dog ++ Cat =15kg=15𝑘𝑔

⇒2(⇒2(Cat ++ Mouse ++ Dog)=8+11+15=34)=8+11+15=34

⇒⇒ Cat ++ Mouse ++ Dog =34:2=17(kg).