I think "like coffee and tea" doesn't mean only coffee and tee.
...I am assuming that since 28 % liked coffee and tea and some % (say x%) like all three things ( coffee, tea and smoke), then to get ONLY coffee and tea % we have to subtract x% from 28 % .Then to get ONLY coffee % , we have to subtract (28 - x) from 48 %.
48% - coffee
(28% - x%) - only coffee and tee
(30% - x%) - only coffee and smoke
x% - coffee, tea and smoke
only coffee = coffee - only coffee and tee - only coffee and smoke - coffee, tea and smoke
only coffee = 48% - (28% - x%) - (30% - x%) - x% = 48% - 28% - 30% + x%
I hope it helps.
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