Perfect inelastic collision

1. The problem statement, all variables and given/known data

Simple question, if a dog jumps on a stationary sled at velocity v.. the dog weighs 20kg.
The velocity after is v/2. What is the weight of the sled

2. Relevant equations
m1v = (m1 + m2)v/2

3. The attempt at a solution
I rearrange this equation and i get m2 = m1.. this is fine.. But what if i use kinetic energy equations, so 1/2mv^2 before should = 1/2mv^2??? In this case i get a different answer , i get m2 = 3m1????

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