A professor reads a greeting card received on his 50 th birthday with

A professor reads a greeting card received on his 50 th birthday with $+2.5 \mathrm{D}$ glasses keeping the card $25 \mathrm{~cm}$ away. Ten years later, he reads his farewell letter with the same glasses but he has 'to keep the letter $50 \mathrm{~cm}$ away. What power of lens should he now use?