I recently had a problem with Numbers installed on my iPad. A few weeks ago when I was installing an update to Numbers the update stalled and would not complete the installation. I was left with a darkened Numbers icon that would not open and I could not delete.
1: The first step was to delete the Numbers app. But simply doing a long press wasn't doing the trick. The only way to delete the app was to go into
Settings > General > Usage > Manage Storage
But there is a trick here. The Numbers icon doesn't show up in the list of apps in this section of the settings menu because it hasn't been fully installed.
Look for the blank icon in the list of apps and select it. It will open up a page like the picture below. Select the option to delete the app.
2: After deleting the app you must perform a hard reset of your device in order to clear out all the junk leftover from the failed installation. To do a hard reset press and hold the power button until the "slide to power off" appears.
3: After restarting your device you can go to the App Store and re-download the application and it should install just fine this time.