A heat engine absorbs 126 kcal of heat and exhausts 77 kcal of heat in each cycle. Calculate the efficiency (as a percentage)

it did 126 -77 = 49 kcal work

100 * 49/126 = 38.9 %

To calculate the efficiency of a heat engine, you need to use the first law of thermodynamics, which states that:

Heat input = Heat output + Work done

In this case, the heat input is given as 126 kcal, and the heat output is given as 77 kcal. We need to first calculate the work done by the heat engine.

Using the first law of thermodynamics, we can rearrange the equation as:

Work done = Heat input - Heat output

Work done = 126 kcal - 77 kcal = 49 kcal.

Now, to calculate the efficiency of the heat engine, we can use the formula:

Efficiency = (Work done / Heat input) * 100%

Efficiency = (49 kcal / 126 kcal) * 100% = 38.89%

Therefore, the efficiency of the heat engine is approximately 38.89%.