If a locomotive uses steam at 100°C, and dumps heat into air at 22.0°C, what is the highest possible efficiency it could have?  No unit?

a locomotive cannot function if the steam is at 100c there would be no surplus energy to run the motor so its efficiency would be zero , the steam has to expand to do work and that means its latent heat must be input and then raised above the boiling  point unless the engins is at high altitude where the boilng point would be lower than 100

a) turn both temperatures into absolute temperatures 373K and 295 K

Now recall your formula for efficiency isn't it (Th - Tl) / Th  = (373-295)/373

= 0.209  or about 21% max.  BUT this is impossible.  It isn't going to be steam at below 100C is it?  Not at atmospheric pressure.

Doesnt it suck that you will soon never be able to ask homework questions on yahoo answers anymore?