1/2 a cup = 4 ounces
Each thermos contains 16 ounces of coffee because 20 - 4 = <<20-4=16>>16
She drinks 32 ounces a day because 2 x 16 = <<2*16=32>>32
She drinks 160 ounces a week because 32 x 5 = <<32*5=160>>160
She starts drinking only 40 ounces because 160 / 4 = <<160/4=40>>40
#### 40