2-4：计算税金

编写⼀个程序，要求⽤户输⼊⼀个美元数量，然后显示出增加 5% 税率后的相应⾦额。

Enter an amount: 100.00
with tax added: $105.00

输⼊的⾦额为⼤于 0 的两位数浮点数。

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

int main()
{
    float x = 0.0f, y;
    scanf("%f", &x);
    y = (1 + 0.05) * x;
    printf("y=%f\n", y);

    return 0;
}

2-7：计算账单

编写一个程序，要求用户输出一个美元数量，然后显示出如何用最少20美元、10美元、5美元和1美元来付款

Enter a dollar amount: 93
$20 bills: 4
$10 bills: 1
$5 bills: 0
$1 bills: 3

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

int main()
{
    int a, b, c, d, e, f, g, h, i;
    a = 20;
    b = 10;
    c = 5;
    d = 1;
    scanf("%d\n", &e);
    f = e / a;
    g = (e - f * a) / b;
    h = (e - a * f - g * b) / c;
    i = (e - a * f - b * g - c * h) / d;
    printf("%d,%d,%d,%d", f, g, h, i);

    return 0;
}

2-8：还贷计算 

编程计算第一、第二、第三个月还贷后剩余的贷款金额

Enter amout of loan: 20000.00
Enter interest rate: 6.0
Enter monthly payment: 386.66

Balance remaining after first payment: $19713.34
Balance remaining after second payment: $19425.25
Balance remaining after third payment: $19135.71

提示：每个月的贷款余额减去还款金额后，还需要加上贷款余额与月利率的乘积。月利率的计算方法是把用户输入的利率转换成百分数再除以12。

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

int main()
{
    float loan, y_inter, m_inter, m_pay;
    printf("enter amout of loan:");
    scanf("%f/n", &loan);
    printf("enter intere rate :");
    scanf("%f/n", &y_inter);
    printf("enter monthly payment:");
    scanf("%f/n", &m_pay);
    m_inter = y_inter / 100 / 12 + 1;
    loan = (loan - m_pay) * m_inter;
    printf("balance remaining after first payment:%f/n", loan);
    loan = (loan - m_pay) * m_inter;
    printf("balance remaining after second payment:%f/n", loan);
    loan = (loan - m_pay) * m_inter;
    printf("balance remaining after third payment:%f/n", loan);
    return 0;
    return 0;
}

3-1 日期格式转换

编写一个程序，以月/日/年即（MM/DD/YY）的格式接受用户录入的日期信息，并以年月日的格式将其显示出来

Enter a date (mm/dd/yyyy): 2/17/2011
You entered the date 20110217

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

int main()
{
    int i, j, n;
    printf("Enter a date (mm/dd/yyyy): ");
    scanf("%d/%d/%d", &i, &j, &n);

    printf("You entered the date %d%.2d%.2d", n, i, j);
	//    system("pause");

    return 0;
}

3-6：分数 加法

修改如下程序（ 3.2 节的 ADDFRAC.C ），使⽤户可以同时输⼊两个分数，中间⽤加号隔开：

Enter two fractions separated by a plus sign: 5/6+3/4
The sum is 38/24

最终代码

#include <stdio.h>
int main(void)
{
    int num1, denom1, num2, denom2, result_num, result_denom;

    printf("Enter two fractions by a plus sign:");
    scanf("%d/%d"  //:: 格式字符串与给定实参不匹配
          "%d/%d",
        &num1, &denom1, &num2, &denom2);
    result_num = num1 * denom2 + num2 * denom1;
    result_denom = denom1 * denom2;
    printf("The sum is %d/%d\n", result_num, result_denom);
    return 0;
}