From this item, we are given that 550 heads appeared out of the 1000 tosses of coins. We calculate for the number of tails by subtracting the number of head from the total number of tosses.
tails = (1000 tosses) - (550 heads)
tails = 450
The probability in this item is calculated by dividing the number of heads from the total number of tosses.
P = 550 / 1000 = 11/20
If the coin is not biased, the probability should have been 10/20 or 1/2 because there are only two sides of the coin and each is likely to occur.