If N denotes number of 8 digit numbers that contains exactly four nines, then unit digit of N is

Step-by-step solution

N= Number of 8 digit number with exactly four 9. aligned N & = Number with [ 1^ st digit is 9 + 1^ st digit is not 9 ] & = ^7 C₃ 9^4+8 ^7 C₄ 9^3 & =9^3 (9 7 6 5 3 2 1 +8 7 6 5 3 2 1 ) & =729 (315+280)=729 595 aligned Hence, unit digit of N is 5 .