Inside file strings.xml
define a String resource like this:
<string name="string_to_format">Amount: %1$f for %2$d days%3$s</string>
Inside your code (assume it inherits from Context) simply do the following:
String formattedString = getString(R.string.string_to_format, floatVar, decimalVar, stringVar);
(In comparison to the answer from LocalPCGuy or Giovanny Farto M. the String.format method is not needed.)