20. A valid ISBN code consists of 10 digits like ISBNa1a2a3a4a5a6a7a8a9a10 such that 10.a1+9.a2+8a3+7a4+6a5+5a6+4a7+3a8+2a9+1a10 must be divisible by 11.
ISBN0111200091 is a valid code. Find the number of pairs of (a5, a9) other than (2, 9) such that the given ISBN is a valid code.

• ISBN0111200091 produces the value 55
  a5=2 and a9=9 will produce the sum = 6*2 + 2*9 =30
  
  so we have to find such numbers which will produce 30
  
  (a5,a9) pair of values will be (3,6) , (4,3) , (5,0 )
• 9 years agoHelpfull: Yes(1) No(0)
• 10
  0,0
  1,1
  2,2
  .
  .
  .
  .
  9,9
• 9 years agoHelpfull: Yes(0) No(2)