a) (250 + 71) × 4 = 250 × 4 + 71 × 4 = 1000 + 284 = 1284
б) 618 × 24 + 618 × 76 = 618 × (24 + 76) = 618 × 100 = 61800
в) 618 × 34 - 518 × 34 = 34 × (618 - 518) = 34 × 100 = 3400