Paul is running with a KE of 600 J if he starts at rest and his final velocity is 4 m/s what is his mass

We use the kinetic energy equation and substitute our data solving for the mass:

ke = (1/2) mv^2

m = 2*ke/v^2

m = 2*600/4^2

m = 1200/16

m = 75 kg

we can substitute directly because the units are homogeneus in the SI system: J, m/s, kg