raju has forgotten his six digit id no. he remembers the following the first two digits are either 1,5 or 2,6 the no. is even and 6 appears twice.if raju uses trial and error process to find his ID no. at the most, how many trials does he need to suceed
If unit digit is not 6, it can be selected in 4 ways. But, number must have 2 6s. The third number required can be chosen in 10 ways. So, the total ways will be 3!/2! (arranging 3 digits) X 10 X 4 = 120
If unit digit is 6, other 3 numbers must have exactly 1 '6'. The other 2 numbers can be selected in 10 X 10 = 100 ways. So, number of arrangements will be 3! X 100 = 600
So, this case must have 720 different arrangements.
You can repeat the same process for case 2 as well which is 2 6 _ _ _ _