Yes, $40 is a reasonable amount to pay for the cab fare.
Explanation
Sheri’s cab fare was $32 and the percentage of gratuity is 20%
So, the amount of gratuity will be:

Thus, the fare of the cab including the gratuity will be:

As Sheri wrote a check to the cab driver for $40 , it means she paying ($40 - $38.40) or $1.60 more to the cab driver. So, the $40 check is a reasonable amount to pay for the cab fare.