2.60/(1-20%)-60 =15 (yuan)

3.2000 * (1+4.77% * 3) = 2286.2 (yuan)

4. s = 3.14 * 0.62 =1.13 (square meter)

5. s = 3.14 (8/2) 2-3.14 * (5/2) 2 = 30.62 (square decimeter)