Final answer:
The efficiency of the solar cells is 20%.
Step-by-step explanation:
To calculate the efficiency of the solar cells, we need to divide the useful energy transferred by the solar cells by the total energy supplied to the solar cells. In this case, the useful energy transferred is 35,000 joules every second and the total energy supplied is 175,000 joules every second. So, the efficiency is calculated as (useful energy / total energy) * 100% = (35,000 / 175,000) * 100% = 20%. Therefore, the efficiency of the solar cells is 20%.