Code:#include <stdio.h> void main(void) { /* ================================== Local Definitions ================================== */ /*Declare a variable for Sales Agent's code number*/ int num; /*Declare variables for sales amount for week 1,2,3,4 , total sales and the commission earned*/ double week1, week2, week3, week4, sum, earn; start: /* Statements */ printf("Please key in Sales Agent's code number>");/* Output statement*/ scanf("%d", &num);/* Input statement*/ fflush(stdin); { if(num>=1000&&num<=9999)/* Define maximum 4 digits for Sales Agent's code number */ goto next; else { printf("Invalid code number! Please key in 4 digits code number.\n"); goto start; } } next: printf("\nPlease key in sales amount for Week 1 (RM)> "); /* Output statement*/ scanf("%lf", &week1); /* Input statement*/ fflush(stdin); /* flush the "enter" or other unrelated characters in the memory */ printf("\nPlease key in sales amount for Week 2 (RM)> "); /* Output statement2*/ scanf("%lf", &week2); /* Input statement*/ fflush(stdin); printf("\nPlease key in sales amount for Week 3 (RM)> ");/* Output statement*/ scanf("%lf", &week3); /* Input statement*/ fflush(stdin); printf("\nPlease key in sales amount for Week 4 (RM)> "); /* Output statement*/ scanf("%lf", &week4); /* Input statement*/ sum= week1 + week2 + week3 + week4;/* Formula for total sales */ printf("\nTotal sales in a month: RM %.2lf", sum);/* Output statement*/ earn= sum *.2; /* Formula for commission earned */ printf("\n\nCommission earned: RM %.2lf\n\n", earn);/* Output statement*/ printf("Press Any Key to continue"); getch(); /* Waiting for any key press to exit the program */ }