The number of four digit telephone numbers having atleast one of their digits repeated is (Note : 0 can be accepted in first place)

Total number of 4 digit telephone numbers=10⁴

Number of 4 digit telephone numbers without repetition=${ }^{10}{P}_4$

The number of four digit telephone numbers having atleast one of their digits repeated is ${10}^4-10{\mathrm{p}}_4$