Having a hard time calculating percentages.

[facade]

Use a sentinel controlled while loop that will ask the user to enter student
grades until a value of -1 is entered. Use a counter variable to count all the
grades that are passing grades, where 70 is the minimum passing grade. If there
are any grades that are out of the range 0-100, present an error messafe to the
user, and do not count that grade as passing. Show the percentage of the valid
grades that are passing.

I don't understand how to calculate the percentage in this. We did not go over it in class.

 /*Use a sentinel controlled while loop that will ask the user to enter student
grades until a value of -1 is entered. Use a counter variable to count all the 
grades that are passing grades, where 70 is the minimum passing grade. If there
are any grades that are out of the range 0-100, present an error messafe to the
user, and do not count that grade as passing. Show the percentage of the valid 
grades that are passing.
*/

#include <stdio.h>
#include <stdlib.h>
// Create a sentinel controlled while loop asking user to enter grade
main() {
	int score; 
		printf("Enter a test score (-1 to quit)");
		scanf_s("%i", &score);

		while (score <= 100 || score <= 1) {
			printf("Enter a test score (-1 to quit):");
			scanf_s("%i", &score);

		}
		// Enter void message for numbers greater than 100 entered.
		void printErrorMessage(int score); {
			if (score > 100);
			printf("That is not a valid grade!\n");
		}
		printf("\nThe percentage of the correct grades are:\n");
	system("pause");
}

[jonnin]

percent is literally just a division by 100 of something. Generally its a part of a total divided by the total.

ok. so what?
so lets count the values that passed and do some stuff: (your trouble is you need more variables to track more info, like how many passed)

blah blah
while(…)
{
if (score >= 70)
numpassing++; //remember to zero initialize these.
totalscores++; //not in the condition..
}

blah blah
double pctpassing = (double)(numpassing)/(double)totalscores; //true %, a number between 0 and 1, multiply by 100 here for a 'human' percent to display it.