1)3/15 - 2 7/25 = 1/5 - 57/25 =(1 * 5 - 57* 1)/25 = 5/25 - 57/25 = - 52/25 = - 2 2/25
2)- 2 2/25 :1 2/25 = - 52/25 : 27/25 = - 52/25 *25/27 = - 52/27 =- 1 25/27
3) - 1 25/27 + 1 4/13 = - 52/27 + 17/13 =(-52 * 13 + 17* 27)/351 = -676/351 + 459/351 = - 217/351