Note: `a/c = a/b xx b/c = 4/5 xx 15/16 = 3/4` `:. (c^2 - a^2)/(c^2 + a^2)` `= (1 - a^2/c^2)/(1 + a^2/c^2)` `= (1 - 9/16)/(1 + 9/16)` `= 7/16 xx 16/25` `= 7/25`

Note: From among the given numbers, 923872 is the only number, whose last 3 digit form 872, which divisible by 8.`

Note: 18962 when divided by 13 leaves the remainder 8. :. Number to be added = (13 - 8) = 5