Final answer:
Engine a's maximum efficiency is inferred to be about 30%, considering it gives off 70% thermal energy. Engine b has a lower maximum efficiency of 25%. Thus, engine a would likely get better gas mileage.
Step-by-step explanation:
Regarding engine a that gives off 70% thermal energy, its maximum efficiency can be calculated using the 1st law of thermodynamics, which correlates to the energy not wasted. However, without additional information about the work output or the heat input, we can only infer that the maximum efficiency represents the percentage of energy not given off as thermal energy. Thus, if 70% is given off as thermal energy, the maximum efficiency could be 30%, assuming all remaining energy is converted to work.
For engine b with maximum efficiency of 25%, we compare it to engine a's inferred maximum efficiency. Since engine b's known maximum efficiency is lower than the inferred 30% efficiency of engine a, engine a is likely to get better gas mileage, as it converts a higher percentage of its fuel energy into work.