a) do not contain the same digit three times = number of strings possible - contain the same digit three times

= 10 x 10 x 10 - 10 x 1 x 1

= 1000 - 10

= 990

b) have exactly two digits that are 4s = 1 x 1 x 9 x 3 = 27

I didn't open the link..

but is it correct ???

= 10 x 10 x 10 - 10 x 1 x 1

= 1000 - 10

= 990

b) have exactly two digits that are 4s = 1 x 1 x 9 x 3 = 27

I didn't open the link..

but is it correct ???