#include <iostream>
#include <string>

int main() {
	float decimal_pounds;
	std::cout << "Enter decimal pounds: ";
	std::cin >> decimal_pounds;

	float pounds, shillings, pennies;
	float remainder;
	remainder = modf(decimal_pounds, &pounds);

	remainder *= 20.0; //convert to shillings
	remainder = modf(remainder, &shillings);
	
	remainder *= 12.0; //convert to pennies
	remainder = modf(remainder, &pennies);
	
	std::cout<<pounds<<" - "<<shillings<<" - "<<pennies<<" - "<<remainder<<std::endl;
	return 0;
}

well I tried your code and it works fine, but I am still confused on how to calculate pence from my code, I really want to learn how to code.

You would write a class that represents pounds, shillings and pence and would provide constructors to initialise it, and provide public methods (and/or free functions taking an object of your class as an argument) to do things with it that would be useful, like adding, subtracting, getting a string representation, etc.

EDIT: An internal representation as an integer number of pennies is one way of doing it, although not if you want farthings or halfpennies to be represented too.

There is, just store the number of pence, and output it in the correct format via a function call.

Then you would wrap that in a class. It's similar to how computers could store the date, they could store the number of milliseconds since some agreed start point, but provide functions to output it in a nice format (and functions to convert various formats for the date into the internal representation). EDIT: And provide other functions to do common tasks with dates.