Energy and basic math (proportionality)

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

A person running in a race has to pick up a mass equal to her own mass. Assuming she can still do the same
amount of work, her speed will be changed by a factor of
a. 0.25
b. 0.50
c. 0.71
d. 1
e. 2

2. Relevant equations


3. The attempt at a solution

Since m is doubled, v2 should be halved. I’m stuck now. But the correct answer is 0.71. Can someone explain how?

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