设后面的三位数是 x
10x + 7 = (7000 + x)/2 + 3
2(10x + 7) = 7000 + x + 6
20x + 14 = x + 7006
20x - x = 7006 - 14
19x = 6992
x = 6992 ÷ 19
x = 368
所以原来四位数是 7368