c++ program runs incorrect else statement - c++

Question

When I run the program, I can't figure out why when I ask for the letter a, b, or c and a different letter is inputted other than those, why is it going to "invalid criteria" instead of "An invalid code has been entered. When I input a number out of the range it goes to "Invalid number has been entered" but not when I'm asking for the letter
#include <iostream>
#include <iomanip>
#include "Header.h"
using namespace std;
int main()
{
head();
int num_1, a, b, c;
char char_1;
float num_2;
num_2 = 28.82;
a = b = c = 0;
cout << "Please enter a number between 10 and 30." << endl;
cin >> num_1;
if (num_1 >= 10 && num_1 <= 30)
{
cout << "Enter the letter a, b, or c." << endl;
cin >> char_1;
if (char_1 == 'a'||'b'||'c')
{
if ((num_1 >= 10 && num_1 <= 20) && (char_1 == 'a'))
{
num_2 = num_2 + .5;
cout << fixed << setprecision(1) << num_2 << endl;
}
else if ((num_1 >= 19 && num_1 <= 30) && (char_1 == 'b'))
{
num_2 = num_2 + .10;
cout << fixed << setprecision(2) << num_2 << endl;
}
else if ((num_1 >= 19 && num_1 <= 30) && (char_1 == 'c'))
{
num_2 = num_2 + .100;
cout << fixed << setprecision(3) << num_2 << endl;
}
else
{
cout << "Invalid criteria" << endl;
}
}
else
cout << "An invalid code has been entered."
}
else
cout << "An invalid number has been entered." << endl;
system("pause");
return 0;
}

Answer 1

The expression:
char_1 == 'a' || 'b' || 'c'
is equivalent to:
char_1 == ('a' || 'b' || 'c')
and therefore first evaluates the logical or of all those letters treated as booleans (all true hence the result is true), then compares that with your variable.
What you need is:
(char_1 == 'a') || (char_1 =='b') || (char_1 =='c')
This checks the character against each of the possibilities and then works out whether any of them were true.