Doris babysits for a total of 5 weekdays x 3 hours/weekday = <<5*3=15>>15 hours
She babysits for a total of 15 + 5 = <<15+5=20>>20 hours in a week.
She earns 24 x $20 = $<<24*20=480>>480 in a week.
So, it will take $1440/$480 = <<1440/480=3>>3 weeks to earn Doris needed amount.
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