Body weight varies directly with weight of water in the body
 thus the equation is          W =  kw      where W = body wt
                                                              k = constant
                                                               w = weight of water
to solve this kind of problem we must first find the value of the
constant  k
thus      63=k42          >    k= 63/42
&            90 = 63/42 w
  w =  90*42/63  =  60kg  of water
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So what is how many kilograms of water are in the 90-kg person??