Answer:
Total people in survey = 10000
Number of people who like folk song only = 3000
Explanation:
0% like none, thus 90% like atleast anyone of them.
Let the total number of people in survey be x.
90x/100 = 70x/100 + 60x/100 - 4000
40x/100 = 4000
x = 400000/40
x = 10000
Total number if people who like folk songs = 70*10000/100
= 7000
Number of people who like folk song only = 7000 - 4000
= 3000