Answer
Please see the work below.
Work Step by Step
We know that percentage composition shows the percentage of molar mass of a substance present in a compound.
We can find the percentage composition of ammonia as follows:
$N=\frac{14.01g\space N}{17.034g\space NH_3}\times 100=82\%$
$H=\frac{3.024g\space H}{17.03g\space NH_3}\times 100=18\%$