It is given that the 5 -digit telephone numbers always start with 67Therefore, there will be as many phone numbers as there are ways of filling 3 vacant places by the digits 0-9, keeping in mind that the digits cannot be repeated.Therefore, the units place can be filled in 8 different ways following which, the tens place can be filled in by any of the remaining 7 digits in 7 different ways, and the hundreds place can be filled in by any of the remaining 6 digits in 6 different ways.Therefore, by multiplication principle, the required number of ways in which 5 -digit telephone numbers can be constructed is 8×7×6=336.