Little Pika who is five and half years old has just
learnt addition. However, he does not know how
to carry. For example, he can add 14 and 5, but he
does not know how to add 14 and 7. How many
pairs of consecutive integers between 1000 and
2000 (both 1000 and 2000 included) can Little
Pika add? (2 marks)
(1) 150 (2) 155 (3) 156
(4) 258 (5) None of the above
My take is 156.
Case 1: Numbers are 1xyp and 1xyq (For example: 1001 and 1002)
(p, q) => (0, 1), (1, 2), (2, 3), (3, 4), (4, 5)
x and y => 0, 1, 2, 3, 4
Total ways = 5*5*5 ways = 125 ways
Case 2: Numbers are 1xy9 and 1x(y+1)0 (For example: 1009 and 1010)
x = 0, 1, 2, 3, 4
(y, y+1) => (0, 1), (1, 2), (2, 3), (3, 4), (4, 5)
Total = 5*5 ways = 25 ways
Case 3: Numbers are 1x99 and 1(x+1)00 (For example: 1099 and 1100)
(x, x+1) => (0, 1), (1, 2), (2, 3), (3, 4), (4, 5)
Total 5 ways
Case 4: (1999, 2000)
Only 1 way here
In all 125 + 25 + 5+ 1 = 156 ways