Vòng 3 - Vòng chung khảo

1.10 277

2. 10 different ways

3. 1

4. 10%

5. 0.14 %

6. 8

7. Password of the mystery box is 1118

If Minh knows the units digit but Binh doesn't know the units digit, Binh cannot guarantee that Minh doesn't.
in the case of 7 or 9 . units
=> Binh knows the unit digit
But you have not determined the password
=> Unit is not 7 or 9
=> Hit 1119 and 1107
If Binh knows both the number of tens and the number of units but the number of tens and the number of units do not coincide, then Binh can determine
password before you Minh said
=> Binh doesn't know tens
=> Minh's friends know dozens
Tens has 4 cases: 0; 1, 2, 3
There are 3 numbers with tens as 3 (1134; 1135; 1138); 2 tens is 2 (1124; 1126); 2 numbers with tens are 0 (1105; 1106)
If tens is 0; 2 or 3, Minh can't determine the password
=> Tens is 1
Only 2 numbers with tens of 1 is 1119 and 1118 is 1119 have been eliminated
=> Password is 1118

Y = 2401 ( In turn, the natural numbers to the power 4 )