$\text{Weight of mangoes + 100 g + 500 g}=\text{1 kg + 2 kg}$
$\Rightarrow \text{Required weight + 600 g}=\text{3 kg}$
$\Rightarrow \text{Required weight}=\text{3 kg - 600 g}$
To move ahead, we need to convert the units. Since we need the final answer in kilograms, let us convert 600 g to kilograms.
        $1\text{kg}=1000\text{g}$
$\Rightarrow \text{600 g}=\frac{600}{1000}\text{kg}$
                $=0.6\text{kg}$
$\therefore \text{Weight of mangoes}=\text{3 kg - 0.6 kg}$
                                        $=\text{2.4 kg}$