Sum of all 4 numbers = Average × 4 = 100 × 4 = 400.<br><br>
&bull; Sum of first 3 numbers = 350<br>
&bull; Fourth number = Total sum − Sum of first 3<br>
&bull; Fourth number = 400 − 350 = 50<br><br>
Final Answer: 50<br>
Check: (sum of first 3 = 350, fourth = 50). Average = (350 + 50) ÷ 4 = 100. ✓