Once you know how to manage percentages, exercises like this will become very easy:
computing a certain percent of a number simply means to multiply that number by a fraction with denominator 100. For example, if you want to compute the 37% of 26, you simply have to compute
[tex]26\cdot \dfrac{37}{100}[/tex]
Let's apply this to your case: we start with an area of 250 sqft.
If we increase this area by 10%, we will have the 110% of the original area, i.e.
[tex]250\cdot \dfrac{110}{100}[/tex]
By the same logic, after the second phase we will have the 120% of the new area, which we just computed:
[tex]\underbrace{\underbrace{\left(250\cdot \dfrac{110}{100}\right)}_{\text{1st phase}}\cdot\dfrac{120}{100}}_{\text{2nd phase}}[/tex]
Now we can do some manipulation/semplification to get
[tex]250\cdot\dfrac{110}{100}\cdot\dfrac{120}{100} = 250\cdot\dfrac{11}{10}\cdot\dfrac{12}{10} = 330[/tex]
