Two short bar magnets with magnetic moments $400 \mathrm{ab}-\mathrm{A}$ $\mathrm{c}{\mathrm{m}}^2\mathrm{ }$and $800 \mathrm{ab}-\mathrm{A}$ $\mathrm{c}{\mathrm{m}}^2\mathrm{ }\mathrm{a}\mathrm{r}\mathrm{e}$ placed with their axis in the same straight line with similar poles facing each other and with their centres at $20 \mathrm{cm}$ from each other. Then the force of repulsion is