11+x-13=40
Challenge of the day!

53

because 11+29=40 40+13=53

so 11+53-13=40

so X=53

i think it might be a but you might want a second opinion

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