title: strange arithmetic with swi-prolog
body: <p>I find the result very strange. Why not 0.3? Can somebody tell me why this result? Is it possible to fix this.  </p>
  
  <pre><code> ?- X is 5.3-5.
      X = 0.2999999999999998.
  
      ?- 
  </code></pre>
  
  <p>My second question is how would I transform from 'hour' notation '13.45' ---->'15.30'  into numbers of hours ? For example the period above calculated 15.30-13.45 would be 1.85. But I need to operate on parts of the hour and not the remainders of the numbers.  Like 15 1/2 - 13 /4, this way is better. I try</p>
  
  <pre><code>?- X is (5.3-5)*100/60.
  X = 0.4999999999999997.
  
  ?- X is (5.3-5)*100//60.
  ERROR: ///2: Type error: `integer' expected, found `29.999999999999982'
  </code></pre>
  
  <p>Any suggestions?</p>