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?
A)35 minutes
B)10.5 minutes
C)350.1 minutes
D)315 minutes

Question

Quizplus · Accepted Answer

Since Mark remembers the first 4 digits of a 7-digit number and knows the last digit is not 0, he needs to guess the remaining 2 digits and the last digit with the constraint. For the 2 unknown digits, each can be any number from 0-9, giving 10 possibilities each. For the last digit, it can be any number from 1-9 (10 possibilities minus the 0), giving 9 possibilities. Thus, the total number of combinations is 10 * 10 * 9 = 900. At 21 seconds per attempt, the total time is 900 * 21 = 18900 seconds. Converting seconds to minutes, 18900 seconds / 60 = 315 minutes.

Solve the Problem. -Mark Can Remember Only the First 4 Digits of His