Juwella read 15 x 2 = <<15*2=30>>30 pages two nights ago.
She read 30 + 5 = <<30+5=35>>35 pages last night.
She read a total of 15 + 30 + 35 = <<15+30+35=80>>80 pages for three nights.
Thus, she needs to read 100 - 80 = <<100-80=20>>20 pages tonight to finish the book.
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