7C4 = 7C3 = 7*6*5 / 1*2*3 = 35
The number of choices is 10*10*9
10*10*9 * 21 = 18,900 seconds
That's only 5 hr 15 min
Evaluate the expression.
Determine the number of combinations of 7 things taken 4 at a time.
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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?
1 answer
1 answer