Aliased frequency from undersampling

A pure sinusoid at $90\,\text{Hz}$ is sampled at $f_s = 100\,\text{Hz}$ with no anti-aliasing filter. Because $90\,\text{Hz}$ is above the Nyquist frequency, it aliases to a lower apparent frequency. What apparent (aliased) frequency, in Hz, will appear in the sampled data?