$29\ 758 = 20\ 000 + 9000 + \boxed{\phantom{700}} + 50 + 8$ What is the missing number in the box?

1. Identify the place value of the missing part in the expanded form

The number $29\ 758$ is expanded as $20\ 000 + 9000 + \text{hundreds} + 50 + 8$. The missing part corresponds to the digit $7$ in the hundreds place.

2. Determine the value of the digit 7

The digit $7$ is in the hundreds place, so its value is $7 \times 100 = 700$.