Madeline spends 4*7=<<4*7=28>>28 hours on homework.
She spends 8*7-56 hours sleeping.
Therefore, she is busy 28+18+56+20=<<28+18+56+20=122>>122 hours per week.
There are 7*24=<<7*24=168>>168 hours per week.
Thus, Madeline has 168-122=<<168-122=46>>46 hours left over.
#### 46