how many times as large is the approximate radius of a mercury atom, 1.50*10^-11 meter, as the approximate radius of a hydrogen atom, 2.5*10^-11
Since both are conveniently $${{\mathtt{x10}}}^{-{\mathtt{11}}}$$, we can just divide 2.5 by 1.5
$${\frac{{\mathtt{2.5}}}{{\mathtt{1.5}}}} = {\frac{{\mathtt{5}}}{{\mathtt{3}}}} = {\mathtt{1.666\: \!666\: \!666\: \!666\: \!666\: \!7}}$$
So a hydrogen atom is 1.6 times larger than a mercury atom.
Since both are conveniently $${{\mathtt{x10}}}^{-{\mathtt{11}}}$$, we can just divide 2.5 by 1.5
$${\frac{{\mathtt{2.5}}}{{\mathtt{1.5}}}} = {\frac{{\mathtt{5}}}{{\mathtt{3}}}} = {\mathtt{1.666\: \!666\: \!666\: \!666\: \!666\: \!7}}$$
So a hydrogen atom is 1.6 times larger than a mercury atom.