To calculate the string efficiency of the suspension insulators, we need to know the voltages across all three insulators. In this case, we only have information about the voltages across the first and second insulators. Without the voltage across the third insulator, we cannot accurately determine the string efficiency.
The string efficiency is defined as the ratio of the voltage across the lowest insulator to the sum of the voltages across all the insulators. Mathematically, it can be represented as:
String Efficiency = (Voltage across Lowest Insulator) / (Sum of Voltages across all Insulators)
Therefore, in order to calculate the string efficiency, we need to know the voltage across the third insulator as well.