Yes, $40 is a reasonable amount to pay for the cab fare.
<em><u>Explanation</u></em>
Sheri’s cab fare was $32 and the percentage of gratuity is 20%
So, the amount of gratuity will be: 
Thus, <u>the fare of the cab including the gratuity</u> will be: 
As Sheri wrote a check to the cab driver for $40 , it means she paying ($40 - $38.40) or <u>$1.60 more to the cab driver</u>. So, the $40 check is a reasonable amount to pay for the cab fare.