[填空题]
A Carnot engine has a lohw,kk;pjkv:n *8mv 1 af :qz:zp8mmx;yf 07upwer operating temperature $T_L\,=\,20^{\circ}\,C$ and an efficiency of 30%. By how many kelvins should the high operating temperature $T_H$ be increased to achieve an efficiency of 40%? $K$
