свойство пропорции: произведение крайних ее членов, равно произведению средних.
1)
х/3 = 28/7
х * 7 = 28 * 3
7х = 84
х = 84 : 7
х = 12
проверка: 12 : 3 = 28 : 7; 4 = 4
2)
х/4 = 25/5
х * 5 = 4 * 25
5х = 100
х = 100 : 5
х = 20
проверка: 20 : 4 = 25 : 5; 5 = 5
3)
70/х = 20/2
70 * 2 = х * 20
140 = 20х
20х = 140
х = 140 : 20
х = 7
проверка: 70 : 7 = 20 : 2; 10 = 10
4)
30/х = 36/6
30 * 6 = х * 36
180 = 36х
36х = 180
х = 180 : 36
х = 5
проверка: 30 : 5 = 36 : 6; 6 = 6
5)
х/5 = 16/4
х * 4 = 5 * 16
4х = 80
х = 80 : 4
х = 20
проверка: 20 : 5 = 16 : 4; 4 = 4
6)
140/х = 40/2
140 * 2 = х * 40
280 = 40х
40х = 280
х = 280 : 40
х = 7
проверка: 140 : 7 = 40 : 2; 20 = 20
58-37 = (50 + 8 ) - ( 30 + 7) = ( 50 - 30 ) + ( 8 - 7) = 20 + 1 = 21
72-51 = ( 70 + 2) - ( 50 + 1) = ( 70 - 50) + ( 2 - 1) = 20 + 1 = 21
71-51 = ( 70 + 1) - ( 50 + 1) = ( 70 - 50) + ( 1 - 1) = 20 + 0 = 20
783-541 = ( 700 + 80 + 3) - ( 500 + 40 + 1) = (700 - 500) + (80 - 40) + (3 - 1) =
= 200 + 40 + 2 = 242
837-615 = ( 800 + 30 + 7) - (600 + 10 + 5 ) = ( 800 - 600) + (30 - 10) + ( 7 - 5) =
= 200 + 20 + 2 = 222
855-54 = ( 800 + 50 + 5) - ( 50 + 4) = 800 + ( 50 - 50) + (5 - 4) =
= 800 + 0 + 1 = 801
342-22 = ( 300 + 40 + 2) - ( 20 + 2) = 300 + ( 40 - 20) + ( 2 - 2) =
= 300 + 20 + 0 = 320
2)
346 - 21 = 325 ( уч.) - во второй школе
346 + 325 = 671 ( уч.) - всего
ответ: 671 ученик.