Let X be the amount Susan had originally. Susan spent 1/5*X in September, 1/4*X in October, and $120 in November.
Susan has X - 1/5*X - 1/4*X - $120 = $540 left.
Combining like terms, we get 11/20*X - $120 = $540
Adding $120 to both sides, we get 11/20*X = $660.
Dividing both sides by 11/20 we get X = $1200.
#### 1200