K -
Local Speed of Sound = 38.954 x (square root of) Outside Air Temp in °K.
1) Find LSS - 93 (kts) divide by 0.15 (Mach Inc.) = 620 kts (LSS!!)
2) Find OAT °K using LSS formula.
620 = 38.954 x sq rt OAT °K
so; sq rt OAT °K = 620 divided by 38.954
so; sq rt OAT °K = 15.162
so; OAT °K = 15.162 squared = 253°K
3) 253°K = ISA +9, so ISA = 253 - 9, = 244°K
Remember, °C = 244 - 273 = -29°C (OAT ISA)
Level that corresponds to 244°K = FL220
On a CRP5...
Line 15 on the inner scale with 93 on the outer scale (above the "10" on the inner, read the LSS of 620 [just to check]).
Go to Airspeed window, above Mach No. Index Arrow read temp of "-20" (ISA + 9°C in your example), so
ISA = -29°C.
Altitude Window: Align Zero Feet with +15°C (ISA) - then read that -29°C equates to 22,000.
Flight Level 220.!!