If for every 10 days worked Andrew earns 1 day of vacation, then over 300 days worked he would have earned 300/10=<<300/10=30>>30 days of vacation
We know he took 5 days off in March and that in September twice as much. That means in September he took 5*2=<<5*2=10>>10 days off
In total, Andrew has used 5 vacation days in March and another 10 in September. Therefore, he has used 5+10=<<5+10=15>>15 vacation days
If Andrew earned 30 days of vacation but has only used 15, then he can still take another 30-15=<<30-15=15>>15 days off
#### 15