let

j = the hours Jean Paul needs to bake the bread himself

m = the hours Marcie needs to bake the bread himself

it takes Marcie 2.5 hours longer than Jean Paul to bake this much bread

m = 2.5 + j

Jean Paul am Marcie can bake a dozen loaves if bread in 3 hours thus their together rate is 12/3 = 4 loaves per hour

1/m + 1/j = 4

plug m = 2.5 + j into 1/m + 1/j = 4

1/(2.5 + j) + 1/j = 4

by solving we find

j = 0.2748 hr

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