unescaped & or unknown entity “&&” javascript

[joanwhite]

This is part of my homework but i can't figure out what is wrong and any help would be appreciated\[code\]var InputNum = prompt("Please enter a number between 50 and 100:", "");if (isNaN(InputNum)) { if (InputNum.match(/one|two|three|four|five|six|seven|eight|nine|ten/)) { alert("while this is a number, it's not really a number to me."); } else { alert(InputNum + " doesn't appear to be a number."); }} else if (InputNum >= 99 && InputNum <= 51); { alert("theat number, " + inputNum + ", is not between 50 and 100.");}document.write("The user gave a number in the range! " + inputNum + "<br>");\[/code\]on line 25 it comes up with the warning that the title states.