Total old pay = 20 × $4,500 = $90,000.<br><br>
New total pay = $90,000 + $15,000 = $105,000.<br>
New number of people = 20 + 1 = 21.<br><br>
New average = $105,000 ÷ 21 = $5,000.<br><br>
Final Answer: $5,000