1. The name of period that has 486 in the given number which is 751 486 is hundreds.
period is considered to a group of places of the digits, like the given number 486 is the group of hundred because the highest number it contains is 400.
=> 400 + 80 + 6
=> 4 hundreds + 8 tens + 6 ones
=> 486
while 751 is the period of hundred thousands.
=> 700 000 + 50 000 + 1 000
=> 751 000