Design a Tips class that calculates the gratuity on a restaurant meal
Inconsistant layout:

#include&lt;iostream&gt;   // No space
#include &lt;string&gt;    // Space


Be consistent in your code. I prefer the version with the space.

Using namespace std

Arrrrr. Don't do that. see: Why is “using namespace std;” considered bad practice?.

Definately never do it in a header file. You are pulliting the namesapce for anybody that includes your code (which can change behavior) and break other peoples code. Don't do it in source files because of the reasons you read in the linked article.

Default argument

      Tips()
      {
            taxRate = .65;
            cout &lt;&lt; "Tax rate is "&lt;&lt;taxRate&lt;&lt;endl;
      }

      Tips(double tax)
      {
                 taxRate = tax;
                 cout &lt;&lt; "Tax rate is"&lt;&lt;taxRate&lt;&lt;endl;
      }


When you see code like this you can usually replace it with a single constructor that has a parameter with a default argument (as they are basically doing the same thing).

      Tips(double tax = 0.065) // Corrected you constant.
      {
           taxRate = tax;
           cout &lt;&lt; "Tax rate is"&lt;&lt;taxRate&lt;&lt;endl;
      }


Be consistent with spacing (white space is your friend)

           cout &lt;&lt; "Tax rate is"&lt;&lt;taxRate&lt;&lt;endl;
                ///            ^^^^^^  ^^^^^        Looking very tight here.

           cout &lt;&lt; "Meal cost + tip rate is "&lt;&lt;tip.computeTip(100,20)&lt;&lt;endl;
                ////                        ^^^^^                   ^^^^


Prefer '\n' over std::endl

std::end forces a flush of the stream. This is never what you actually want. The automated flushing of the stream will work much better than manual flushing.

Prefer to use the initializer list.

      Tips(double tax = 0.065) // Corrected you constant.
           :   taxRate(tax)
      {
           cout &lt;&lt; "Tax rate is"&lt;&lt; taxRate &lt;&lt; endl;
      }


Arithmetic issue

If the billAmount is mealCost * (1 + taxRate) then this is not the formula to calculate meal cost.

            double mealCost = billAmount - billAmount*(taxRate);