Four Fair Coins Are Flipped $X _ { 1 }$ Corresponding to Sum =0

Essay

Four fair coins are flipped.Each coin tail is recorded as a 0 and a head as a 1 and the source symbol is the sum, with $X _ { 1 }$ corresponding to sum =0, $X _ { 2 }$ to sum = 1, etc.The probability of the source
producing $X _ { 3 }$ equals _______.