My base class has constructor and derive class has default constructor, but it can't compile. Can anyone give me a proper explan

Question

//code is given

#include <iostream>
using namespace std;

class Mother
{
public:
Mother(int a):age(a)
{
cout<< age;
}

void sayHi() {
cout << "Hi";
}
private:
int age;
};

class Daughter: public Mother
{
public:
Daughter() {};
};

int main() {
Daughter d;
Mother m(30);
d.sayHi();
}
//ERROR: no matching function for Mother::Mother()

Answer 1

Add an empty default constructor in 'Mother' class.

Mother(){}

I can't provide the technical details, OOP is one of my many weaknesses. I'll leave that to those who are more knowledgeable and stronger in OOP.

Answer 2

Give me a proper explanation!? It's not a clear proposition, though program runs as @lpang instruct. But why use double constructor in Mother class?
//code give below
class Mother
{
public:

Mother(int a):age(a)
{
cout << age<<endl;
cout<< "age mother\n";
}
Mother(){}; // Why use another constructor for running
void sayHi() {
cout << "Hi";
}
private:
int age;
};

class Daughter: public Mother
{
public:
//Default constructor
Daughter() {
cout <<"daughter age\n";
}
};

int main() {
Daughter d; //Why not it inherit parameter base constructor
Mother m(14);
d.sayHi();
}