Total customers are = 360
Number of polled people who purchased ice cream = 260
Number of polled people who purchased both ice cream and yogurt = 250
Hence, people who purchased only ice cream are=
And
are people who purchased yogurt.
So, people who purchased yogurt only = 
-4 = 8m + 18n
-18n = 8m + 4
/-18 /-18 /-18
n = 8m/-18 + 4/-18
I'm not sure so yeah