Since there are 30 days in a month and Jill makes $10 a day her first month, she makes 30*10= <<30*10=300>>300 dollars her first month.
She makes twice the amount of the previous month's $300 in her second month, so she makes 2*300=<<300*2=600>>600 dollars her second month.
She makes the same amount per day the third month, but only works half the number of days, so she makes 600/2= <<600/2=300>>300 dollars a month her third month.
In total, she makes 300+300+600= <<300+300+600=1200>>1,200 dollars
#### 1,200