On Monday, he spent half of $80 leaving him with $80-(1/2)*$80 = $<<80-(1/2)*80=40>>40
On Tuesday, he spent 1/5th of $40 leaving $40-(1/5)*40 = $<<40-(1/5)*40=32>>32
On Wednesday, he spent 3/8ths of $32 leaving $32-(3/8)*$32 = $20
#### 20