Given that earning for 1 hour by weeding neightborhoods garden = $15
Then earning for 2 hours by weeding neightborhoods garden = $15*2= $30
Then earning for 3 hours by weeding neightborhoods garden = $15*3
Using same pattern earning for t hours by weeding neightborhoods garden = $15*t= $15t
Not it says that earning is doubled means you are getting 2*15 or $30 per hour
then new earning will be $30t in place of $15t
Hence final answer is $30t.