Avery in 5 minutes sweeps 1 deck, so in 1 minute she sweeps 1/5 of the deck
Madeline in 7 minutes sweeps 1 deck so in 1 minute she sweeps 1/7 of the deck.
1/5 + 1/7 = 7/35 + 5/35 = 12/35
Together they will sweep 1 deck in x minutes : 1/x
12/35 = 1/x =
X = 35/12 = 2.92 minutes. ( round the answer as needed)