I posted this once, then pulled it because Jane Doh is almost certainly an anti-sciolist, but for those who are genuinely interested, the speed of sound expressed in EAS is independent of temperature. You only need to know pressure altitude and then multiply by Mach Number to get the flight EAS.
Values:
Press.Alt Speed of sound
65000 ft - 156.02 kts EAS
70000 ft - 138.35 kts EAS
75000 ft - 122.69 kts EAS
80000 ft - 108.8 kts EAS