What will be the ratio of the weight of a body on earth to Jupiter? If the gravitational acceleration at Jupiter is $2.5$ times at earth.

The ratio of body weight on Earth to Jupiter is$0.4$.
The formula to calculate weight is $\mathrm{mass}✕\mathrm{acceleration}$ due to gravity.
To calculate the ratio of weight -
$\frac{{\mathrm{W}}_{\mathrm{e}}}{{\mathrm{W}}_{\mathrm{j}}}=\mathrm{ }\frac{{\mathrm{mg}}_{\mathrm{e}}}{{\mathrm{mg}}_{\mathrm{j}}}$
Where, $m$ is the body's mass, 
          $W_e$  is  its weight on Earth,
           $W_j$ is its weight on Jupiter,
            $g_e$ is its gravitational acceleration on Earth 
            $g_j$ is its gravitational acceleration on Jupiter
⇒  $\frac{{\mathrm{W}}_{\mathrm{e}}}{{\mathrm{W}}_{\mathrm{j}}}=\mathrm{ }\frac{{\mathrm{mg}}_{\mathrm{e}}}{{\mathrm{mg}}_{\mathrm{j}}}$
$\Rightarrow \mathrm{ }\frac{{\mathrm{W}}_{\mathrm{e}}}{{\mathrm{W}}_{\mathrm{j}}}=\mathrm{ }\frac{{\mathrm{mg}}_{\mathrm{e}}}{2.5{\mathrm{mg}}_{\mathrm{e}}}$
$\Rightarrow \mathrm{ }\frac{{\mathrm{W}}_{\mathrm{e}}}{{\mathrm{W}}_{\mathrm{j}}}=\mathrm{ }\frac{1}{2.5}$
⇒$\mathrm{ }\frac{{\mathrm{W}}_{\mathrm{e}}}{{\mathrm{W}}_{\mathrm{j}}}=\mathrm{ }0.4$