Yesterday, Lara collected 50 + 30 = <<50+30=80>>80 aluminum cans.
So Sarah and Lara collected a total of 50 + 80 = <<50+80=130>>130 aluminum cans yesterday.
Today, Sarah and Lara collected a total of 40 + 70 = <<40+70=110>>110 aluminum cans.
Thus, they collected 130 - 110 = <<130-110=20>>20 fewer aluminum cans today than yesterday.
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