Task 3. Write down all four-digit numbers that are written with two fives and two sevens. Calculate the difference between the largest and smallest number of the obtained numbers.
Solution. The rearranged numbers are: 5577, 5757, 5775, 7557, 7575, 7755. The largest of these six numbers is 7755, and the smallest is 5577. Their difference is $7755-5577=2178$.