I am not sure why you would be constrained like that but it appears that you must pass only the subscript which means the data must be globally accessible. As mentioned previously, the value fourteen should NOT be hard-coded. Here is one way you can deal with these things :
#include <iostream>
const int ScoreCount = 14;
int Scores[ ScoreCount ] = { 90, 61, 74, 42, 83, 51, 71, 83, 98, 87, 94, 68, 44, 66 };
void func( int index )
{
if( Scores[ index ] >= 70 )
Scores[ index ] += 15;
}
int main()
{
for( int i = 0; i < ScoreCount; i++ )
{
func( i );
}
std::cout << " The new adjusted grade scores are: ";
for( int i = 0; i < ScoreCount; i++ )
{
std::cout << i << " : " << Scores[ i ] << std::endl;
}
return 0;
}