let

c = the hour that Cindy needs to mow the lawn alone thus her rate is 1/c lawns/hr

j = the hours that Jim need to mow the lawn alone thus his rate is 1/j lawns/hr

together they mow the lawn in 8 hour thus their rate is 1/8 lawns/hr

1/c + 1/j = 1/8

Cindy takes four times as long as Jim

c = 4j

plug c = 4j into 1/c + 1/j = 1/8 we get

1/(4j) + 1/j = 1/8

by solving we find

j = 10 hr

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Jim can mow the lawn alone in 10 hours.