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Solve the problem.

Mark can remember only the first 4 digits of his friend's phone number. He also knows that the number has 7 digits and that the last digit is not a 0. If Mark were to dial all of the possible numbers and if it takes him 21 seconds to try each one, how long would it take to try every possibility?

Sagot :

1234xxx0  1`= 21 x3 2= 21x 3 3 =21x3 4= 21x 3 5=21x3  6=21x3 7=21x3 8=21x3 9=21 x3  irt will take him 588 secs