Simple JavaScript tip calculator
Eliminate code duplication by using function parameters (numbers in function name is usually a bad smell )
Separate calculating/processing data and DOM manipulations


My suggested solution:

function calcTip(bill, percent) {
    return bill * percent / 100;
}

function getMessage(tip, percent) {
    return 'A ' + percent + ' percent tip would equal $' + tip;
}

function updateFunc(billInput, tipSuggestion) {
    // Here I use closure to save billInput and tipSuggestion
    return function(percent) {
        var bill = parseFloat(billInput.value) || 0;
        tipSuggestion.innerHTML = getMessage(calcTip(bill, percent), percent);
    }
}

var tenPerBtn = document.querySelector('.tenPercent');
var fifteenPerBtn = document.querySelector('.fifteenPercent');
var twentyPerBtn = document.querySelector('.twentyPercent');
var customPerBtn = document.querySelector('.customTipBtn');
var bill = document.getElementById('billInput');
var tipSuggestion = document.getElementById('tipAmount');

var update = updateFunc(bill, tipSuggestion);

tenPerBtn.addEventListener('click', function() { update(10); });
fifteenPerBtn.addEventListener('click', function() { update(15); });
twentyPerBtn.addEventListener('click', function() { update(20); });
customPerBtn.addEventListener('click', function() { update(document.querySelector('#customTip').value); });