B, after inserting the number n in the middle, the three digits are 100a.

10n

b

So 100a

10n

b=9( 10a

b)

Namely 5a

When 5n=4b, n=4b/5-a is obtained.

So b can only take 5, and n = 4-a.

A takes 1, 2, 3 and 4 in turn; N takes 3, 2, 1, 0 in turn.

So the original two digits are 15, 25, 35 and 45.

.