Let the speed of the current be c. I suspect the boat’s speed is measured in still water so the effective speed against the current is 20-c and with the current it’s 20+c, so it takes longer to travel upstream.
Time=distance/speed. The distance is constant so we can write distance=60(20-c)=30(20+c).
2(20-c)=20+c, 40-2c=20+c, 3c=20, c=20/3mph=6⅔mph. The speed of the current cannot vary.