The numbers may seem intimidating but in the end it is just a division:
Divide mercury by hydrogen:
1.5x10⁻¹⁰ / 2.5 x 10⁻¹¹ = (1.5 / 2.5) x (10⁻¹⁰ / 10⁻¹¹) = 0.6 x 10⁻¹⁰⁻⁻¹¹ = 0.6x10 = 6
What I did is I turned it into two divisions, so I could deal with the exponents separately. If you remember that dividing equal numbers with exponents means subtracting those exponents, you see that 10⁻¹⁰ / 10⁻¹¹ is just 10.
1.75x10⁻¹⁰ / 2.5 x 10⁻¹¹ = 7.
Another way of doing this, is seeing that the exponents differ by 1. So the mercury and uranium are ten times bigger. That simplifies the divisions to
15 / 2.5 = 6
and
17.5 / 2.5 = 7
