Ross and Annie do chores at their home acreage. Ross mows the acreage lawn every 12 days, and Annie bathes the dog every 21 days. If Ross and Annie do their chores today, how many days will pass before they both do their chores again on the same day?

84 days will pay before they both do their chores again on the same day.
Ross mows the lawn   Annie bathes the dog
12 x 1 = 12   21 x 1 = 21
12 x 2 = 24   21 x 2 =  42
12 x 3 = 36   21 x 3 =  63
12 x 4 = 48    21 x 4 = 84
12 x 5 = 60    21 x 5 = 105
12 x 6 = 72   21 x 6 = 126
12 x 7 = 84   21 x 7 = 147
12 x 8 = 96 21 x 8 = 168
12 x 9 = 108 21 x 9 = 189
12 x 10 = 120   21 x 10 = 210