You almost got it, change it to like this
var grades = [100, 90, 100, 50, 80, 60];
function countFailing(gradesArr){
var counter = 0;
for (var i = 0; i < gradesArr.length; i++){
if (gradesArr[i] < 70 )
counter++;
}
return counter;
}
alert(countFailing(grades));
solved How to make a function take in an array of numbers and give the ones less than 70?