A phone number contains 7 digits. How many different numbers can be made using the digits 0–9 if the first digit is not 0 and all of the digits can be repeated
In 7 digit phone number, the first number cannot be 0
So only 1 to 9 are used to get the first digit. 9 numbers can be used
other digits can use number from 0 to 9. 10 number can be used
By virtue of the Pythagorean Theorem, in a right triangle the sum of the squares of the smaller two sides equals the square of the largest side. Only 9, 12, and 15 fit this rule.
