Which is taller the gateway arch 630 feet
tall or the space needle 605 feet tall how do you know

The Gateway Arch is taller with 630 feet compared to the Space Needle which is 605 feet. The higher number is the taller or bigger one. Compare the two numbers:
630
605
6 is in common in the hundreds place so that's the same
In the tens place, 3 is bigger than 0. Since 3 is bigger than 0, anything to the right of the bigger number is now considered "not counting", so 630 is bigger.