Store receives 1000 shirts and pays $9000
$$1\ shirt=\frac{$9000}{1000}=$9$$

For 1 month, the store sells 80% above the cost.
Selling price of 1 shirt for 1 month = [80% of $9] + $9
$$=\left[\frac{80}{100}\cdot9\right]+9$$
$$=7.2+9$$
$$=$16.2$$
Thus, 75% of the shirts were sold in the first month.
$$i.e\ 75\%\ of\ 1000\ =\ \frac{75}{100}\cdot1000=750$$
$$So,\ 750shirts\ were\ sold\ for\ =\ $16.2\cdot750$$
$$=\ $12150$$
After this month, the store sells the shirt at 20% above the cost.
Selling price of 1 shirt now become = [20% of $9] + $9
$$\left[\frac{20}{100}\cdot$9\right]+9$$ $$ $1.8+$9= $10.8$$

50% of the remaining shirts were sold during this period.
Remaining shirt = 1000 - 750 =250
$$50\%\ of\ 250\ =\ \frac{50}{100}\cdot250\ =125shirts$$
125 shirts were sold for = $10.8 x 125 = $1.350

Therefore, Total gross income income = $12,150 + $1,350
$$=$13,500$$