If $20\%$ of a number is $50$, what is $125\%$ of that number?

Let the number be $x$.
According to the question:

$$20\%\text{ of }x=50$$

$$\frac{20}{100}\times x=50$$

$$x=\frac{50\times 100}{20}=250$$

Now, we need to find $125\%$ of this number ($250$):

$$125\%\text{ of }250=\frac{125}{100}\times 250$$

$$=1.25\times 250=312.5$$