Identify fuming sulphuric acid

Fuming sulphuric acid is called oleum.

Write its chemical formula

Oleum is commonly represented as: $\mathrm{H_2S_2O_7}$ It can also be thought of as: $\mathrm{H_2SO_4 + SO_3 \rightarrow H_2S_2O_7}$

Count the oxygen atoms

In $\mathrm{H_2S_2O_7}$, the subscript of oxygen is $7$.

Therefore, the number of oxygen atoms is: $7$

Compare with stored correct answer

Stored correct answer = $7$

Our derived answer = $7$

So, they match.