abcd :9=>rest 7
abcd=1000a+100b+10c+d=(999a+99b+9c)+(a+b+c+d)
abcd=9(111a+11b+c)+(a+b+c+d)
abcd:9=(111a+11b+c), rest (a+b+c+d)
=> a+b+c+d=7
dbca =1000d+100b+10c+a=(999d+99b+9c)+(a+b+c+d)
dbca =9(111d+11b+c)+(a+b+c+d)
dbca:9=(111d+11b+c), rest (a+b+c+d)
=> a+b+c+d=7
restul impartirii lui dbca la 9 este egal cu 7
