= Solution
The dominant <semileptonic decay> is $t\to bW^+\to b\ell^+\nu_\ell$. Replacing $b$ by $s$ or $d$ weights the <Probability amplitude>[amplitude] by $V_{ts}$ or $V_{td}$ rather than $V_{tb}$. Neglecting the down-type <masses>,
$$
\boxed{B(t\to bW^+)\simeq\frac{|V_{tb}|^2}{|V_{td}|^2+|V_{ts}|^2+|V_{tb}|^2}=0.9983}
$$
with the source's rounded <matrix> entries. For the $W$, three leptonic widths and two open <Up quark>[up-type quark] rows with three colours give nine common width units. Thus, ignoring <QCD> and final-mass corrections,
$$
\boxed{B(t\to b\ell^+\nu_\ell)\simeq\frac{0.9983}{9}\simeq11.1\%\quad\text{per lepton flavour}.}
$$
Summing all three <Flavor eigenstates>[flavours] gives approximately one third; summing only <electron> and <muon> modes gives approximately $22.2\%$. These are per-top <branching ratios>, not the separate <branching ratio> of a specified two-top event topology.
An <Lepton isolation>[isolated] high-transverse-momentum charged lepton, <missing transverse momentum> from the <neutrino>, and a tagged <Bottom-quark jet tagging>[bottom jet] form a much cleaner signature than an entirely hadronic final state amid <QCD> <jets>. <Electrons> and <muons> are the easiest charged-lepton <Flavor eigenstates>[flavours] to identify cleanly. <Tau lepton>[Tau] decays add <neutrinos> and often hadrons, making identification and reconstruction more difficult. The figure contains the <quark>/lepton-level <Charged current>[charged-current] diagram as well as the rare-loop examples used below.
\Image[/past-exam-of-the-particle-physics-course-of-the-university-of-oxford/2017-top-decay-flavour-channels.png]
{title=Dominant semileptonic top decay and representative flavour-changing neutral-current loop diagrams}
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