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A town N is 340 k due west of town G and town K is due west of town N. A helicopter Zebra left G for K at 9.00 a.m. Another helicopter Buffalo left N for K at 11.00 a.m. Helicopter Buffalo travelled at an average speed of 20km/h faster than Zebra. If both helicopters reached K at 12.30 p.m. , find the speed of helicopter Buffalo.