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Instead of all the if-else statements, you could use a switch statement, like so:

switch (x) {
  case 15:
    int y = z/10000000000000;
    if (y==34||y==37)
    ...
    ...
    ...
    break;
  case 13:
...
...
...
  default:
    printf("INVALID\n");
}


Also to make your code look cleaner, eliminate any curly brackets around if  statements that execute only one statement, for example,

if (l%10==0)
{
        return true;
}


could be:

if (l%10==0) return true;


Additionally, instead of

if (y==51||y==52||y==53||y==54||y==55)


use

if (y&gt;=51&amp;&amp;y&lt;=55)