very simple,
suggest the first 3 digits are x,y,z
so 1. 100x+10y+z
2. (1)x80=8000x+800y+80z
3. (2)+1=8000x+800y+80z+1
4. (3)x250=2000000x+200000y+20000z+250
suggest the last 4 digits are a, b, c, d
then comes to be 2000000x+200000y+20000z+250+1000a+100b+10c+d
then again 2000000x+200000y+20000z+250+2(1000a+100b+10c+d)
then minus 250 to be 2000000x+200000y+20000z+2000a+200b+20c+2d
then divided all by 2, to be 1000000x+100000y+10000z+1000a+100b+10c+d
That's what your phone # is, since you suppose it was a 7-digit number.
lol, bang wo jia ge sheng wang ba!
