Jess’ second stack was 7 blocks + 5 blocks = <<7+5=12>>12 blocks.
Jess’ third stack was 12 blocks + 7 blocks = <<12+7=19>>19 blocks high.
Jess’ sister knocked down the second tower and knocked 12 blocks – 2 = <<12-2=10>>10 blocks knocked over.
Jess’ sister knocked down the final tower and knocked 19 blocks – 3 = <<19-3=16>>16 blocks knocked over.
Total blocks knocked over was 7 blocks + 10 + 16 = <<7+10+16=33>>33 blocks.
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