To find the ratios, both quantities should be in same unit
$$15kg$$ to $$210g$$
$$\Rightarrow$$ $$15\times 1000g$$ to $$210g$$ ($$\because$$ $$1kg=1000g$$)
$$\Rightarrow$$ $$15000g$$ to $$210g$$
Thus the ratio is $$=\cfrac{15000}{210}=\cfrac{500}{7}=500:7$$