How do you double the efficiency of a Carnot heat engine?

Question

It is desired to double the efficiency of a Carnot heat engine from 30% by raising its temperature of heat addition, while keeping the temperature of heat rejection constant. What percent increase in high temperature is required?

Answer 1

efficiency is 1-Tc/Th = 30%
Then Tc/Th = .7

We want .6 = 1-Tc/Th, or Tc/Th = .4

Since only Th changes Thnew/Thold = .7/.4 = 1.75