A number consisting of 2 identical digits, say
xx is multiplied by
99.
Write down the product P, given its tens digit (i.e. 3rd from the left) is y.
P&P only.
(In reply to
Solution (spoiler) by armando)
Building on Armando's solution
P= (9-y,8-y,y,y+1)
We get P =
1000*(9-y) + 100*(8-y) + 10y + y+1
= 9801 - 1089y