c# - Dollar Object intricacies -

i reading kent beck's book "test driven development: example." in book, there coding example:

public class dollar {     public int _amount;     public dollar(int amount)     {         _amount = amount;     }     public dollar times(int multiplier)     {         return new dollar(_amount *= multiplier);     } }  [testmethod] public void testmethod2() {                             dollar 5 = new dollar(5);     dollar product = five.times(2);     assert.areequal(10, product._amount);     product = five.times(3);     assert.areequal(15, product._amount); } 

according kent, second dollar object: "product," necessary in order preserve original "five" dollar object; however, second assertion returns false because product._amount equal 30. have been unable find errata on text. how kent's code above need change in order have second assertion equal true or 15 == 15? book's example flawed? why product._amount in second assertion not equal 15?

as has been said in comments, this:

public dollar times(int multiplier) {     return new dollar(_amount *= multiplier); } 

should be:

public dollar times(int multiplier) {     return new dollar(_amount * multiplier); } 

in original form, *= operator modifies _amount variable of instance of dollar class times called on. removing = changes operation reads value of _amount variable , uses in calculation.