Functions Practise

#include<stdio.h>
#include<conio.h>
#include<stdlib.h>
#include<Windows.h>


double areaRightTriangle(float base, float height);
int isValid(int number);
float converttoCelsius(float Farenheit);
int getSeconds(int hours, int minutes, int seconds);
int isMultiple(int num1, int num2);
void countdownTimer(int seconds);

void main()
{
    int op_int1, op_int2, op_int3=1;
    float op_f1, op_f2;

    while (op_int3)
    {
        system("cls");
        op_int3 = 1;
        printf("\nChoose a program:\n\n1.\tCalculate Area(Right Triangle)\n2.\tIs my number part of the set 1-100?\n3.\tConvert Farenheit to Celsius\n4.\tConvert hours, minutes, seconds to seconds\n5.\tIs my 2nd number a multiple of the 1st?\n0.\tExit Program\n");
        scanf_s("%d", &op_int3);
        system("cls");
        if (!(op_int3)) break;
        else if (op_int3 == 1)
        {
            printf("\nEnter base of the right triangle: ");
            scanf_s("%f", &op_f1);
            printf("\nEnter altitude/height of the right triangle: ");
            scanf_s("%f", &op_f2);
            printf("The area of the right triangle is %.2lf\nReturning to main menu in \n", areaRightTriangle(op_f1, op_f2));
            countdownTimer(10);

        }
        else if (op_int3 == 2)
        {
            printf("\nEnter your number: ");
            scanf_s("%d", &op_int1);
            if (isValid(op_int1))
            {
                printf("\n\nYour number %d is within 1-100\nReturning to main menu in \n", op_int1);
                countdownTimer(10);
            }
            else
            {
                printf("\n\nYour number %d is not within 1-100\nReturning to main menu in \n", op_int1);
                countdownTimer(10);
            }
        }
        else if (op_int3 == 3)
        {
            printf("\n\nPlease enter the temperature in Farenheit: ");
            scanf_s("%f", &op_f1);
            printf("\n\nThe temperature %.2f in Celsius is %.2f\nReturning to main menu in \n",op_f1,converttoCelsius(op_f1));
            countdownTimer(10);
        }
        else if (op_int3 == 4)
        {
            printf("\nEnter Hours: ");
            scanf_s("%d", &op_int1);
            printf("\nEnter Minutes: ");
            scanf_s("%d", &op_int2);
            printf("\nEnter Seconds: ");
            scanf_s("%d", &op_int3);
            printf("\n\nThe total of %d hours, %d minutes, and %d seconds is %d. \nReturning to main menu in \n", op_int1, op_int2, op_int3, getSeconds(op_int1, op_int2, op_int3));
            countdownTimer(10);
        }
        else if (op_int3 == 5)
        {
            printf("\nEnter first number: ");
            scanf_s("%d", &op_int1);
            printf("\nEnter second number: ");
            scanf_s("%d", &op_int2);
            if (isMultiple(op_int1, op_int2))
            {
                printf("\n\nThe number %d is a multiple of %d\nReturning to main menu in \n", op_int2, op_int1);
                countdownTimer(10);
            }
            else
            {
                printf("\n\nThe number %d is not a multiple of %d\nReturning to main menu in \n", op_int2, op_int1);
                countdownTimer(10);
            }
        }
        else
        {
            printf("\n\nInvalid entry. Please enter a number from 1-5\nReturning to main menu in \n");
            countdownTimer(10);
        }
    }
}

double areaRightTriangle(float base, float height)
{
    return (0.5*base*height);
}

int isValid(int number)//1-100==1, else 0;
{
    if ((number <= 100) && (number >= 1)) return 1;
    else return 0;
}

float converttoCelsius(float farenheit)
{
    return ((float) 5 / 9 * (farenheit - 32));
}

int getSeconds(int hours, int minutes, int seconds)
{
    return(60 * 60 * hours + 60 * minutes + seconds);
}

int isMultiple(int num1, int num2)
{
    if (!(num2%num1))
        return 1;
    else return 0;
}

void countdownTimer(int seconds)
{
    for (seconds; seconds; seconds--)
    {
        printf("%2d second/s\r", seconds);
        Sleep(1000);
    }
}