###Human: Ursula is working at a marketing firm. She created a 30-second long commercial. Her boss told her that this commercial is too long to air and told her to shorten the commercial by 30%. How long will this commercial be after Ursula makes the desired changes?, ### Assistant: Ursula's boss want's the commercial to be 30/100 * 30 = 9 seconds shorter.
After Ursula makes the desired changes, the ad is going to be 30 - 9 = 21 seconds long. 
 So the answer is :  21