= Solution
At leading order, neglect the <transverse momentum> of the incoming <partons> and of the produced <On shell>[on-shell] boson. In the hadron <Center-of-momentum frame>[centre-of-mass frame],
$$
p_+=\frac{\sqrt s}{2}(x_+,0,0,x_+),\qquad
p_-=\frac{\sqrt s}{2}(x_-,0,0,-x_-).
$$
Then $m_W^2=(p_++p_-)^2=x_+x_-s$, and
$$
y=\frac12\ln\frac{x_+}{x_-}.
$$
Solving these product and ratio equations gives
$$
\boxed{x_+=\sqrt\tau e^y,\qquad x_-=\sqrt\tau e^{-y},\qquad \sqrt\tau=\frac{80.4}{2000}=0.0402.}
$$
Over $-2<y<2$, each <parton momentum fraction>[momentum fraction] ranges between
$$
\boxed{0.00544<x_\pm<0.2970.}
$$
They are anticorrelated: increasing $y$ increases $x_+$ while decreasing $x_-$. Higher-order recoil changes the simple two-to-one <kinematics>.
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