the problem is best solved with Venn diagram but since we don't have the option to draw we will solve it another way

12% own an ipad and iPhone

20% of you classmates own iPad (this percentage includes the 12% that own an ipad and iPhone) thus

20 - 12 = 8% own iPad only

p% own iPhone only

12 + 8 + p + 74 = 100

p = 100 - 94

p = 6% own iPhone only.