Top speed is a speed one eighth more than standard speed.

Let

'v1' be the standard speed of the sailfish then the top speed will be v1 + (1/8)*v1

'v2' be the standard speed of the peregrine falcon then the top speed will be v2 + (1/8)*v2

v1 = ((1/5)*(v2 + (1/8)v2)) (sailfish can swim up to 1mile per hour less than one fifth the top speed of a peregrine falcon)

The tops speed of sailfish is v1 + (1/8)*v1 thus we have

v1 + (1/8)*v1 = v1 (1 + 1/8) = ((1/5)*(v2 + (1/8)v2)) (9/8) = (1/5) v2 (9/8) (9/8) = (81/320) v2

The top speed of the sailfish is (81/320) v2 miles per hour.