Andrea sold 16 x 2 = <<16*2=32>>32 tickets.
Half the number of tickets Jude sold is 16/2 = <<16/2=8>>8.
So, Sandra sold 8 + 4 = <<8+4=12>>12 tickets.
Thus, Andrea, Jude, and Sandra sold a total of 32 + 16 + 12 = <<32+16+12=60>>60 tickets.
Therefore, 100 - 60 = <<100-60=40>>40 tickets need to be sold.
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