The answer is .30
Since 30 students responded high interest so the relative frequency of students with high interest is 
30/100=0.30 
So required relative frequency is 30%. 
Total frequencies: 3+7+11+22+40+24+9+4=120 
The total frequencies of those who sales 1600 or more is 24+9+4=37 
So required relative frequency will be 
37/120 = 0.308