the 8 in 8,420 is in the thousands place so it's value is 8 thousand.
the 8 in 6,870 is in the hundreds place so it's value is 8 hundred.
divide 8000 by 800 to find the answer:
8000 / 800 = 10
the 8 in 8,420 is 10 times greater than the 8 in 6,870