As shown in the figure, fold a rectangular piece of paper ABCD along AF to make point B fall at B ′. If ∠ ADB = 20 °, then how many degrees should ∠ BAF be to make ab ′‖ BD?

∵ the rectangular paper piece ABCD is folded along AF, so that the B point falls at B ′, ∵ B ′ - AF = ∵ BAF, ∵ ab ′ - BD, ∵ B ′ - ad = ∵ ADB = 20 °, ∵ B ′ - AB = 20 ° + 90 ° = 110 °, ∵ BAF = 110 °△ 2 = 55 °. The ab ′ - BD can be made only when the ∵ BAF is 55 degrees

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