The interest on the loan is 0.1*1200=$<<0.1*1200=120>>120

The total amount Jack needs to pay back is 1200+120=<<1200+120=1320>>1320

# Answer

1320
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
Jack and Jill are friends who borrow from each other often. Last week Jack borrowed  $1200 from Jill, which he promised to pay back with an interest of 10%. How much will Jack pay back?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
1320
Jack and Jill are friends who borrow from each other often. Last week Jack borrowed  $1200 from Jill, which he promised to pay back with an interest of 10%. How much will Jack pay back?
train