MTH LIBERAL ARTS
posted by Jamie
Evaluate the expression.
Determine the number of combinations of 7 things taken 4 at a time.
&
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?

Steve
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
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