Look at the chart "Number of Units Sold as a Percent of Number of Units in Stock":
The number of television sets sold by Store R last month was approximately 50% $(0.5)$ of its number of television sets in stock, which is: $0.5*40 = 20$.
The number of television sets sold by Store T last month was approximately 90% $(0.9)$ of its number of television sets in stock, which is: 0.9*50 = 45.
And now, the original question is equivalent to: "20 is approximately what percent less than $45$".
To compute A is what percent less than B, we compute
$$
\frac{\mathrm{difference}}{\mathrm{base}}
$$
,
where difference is B-A (since A is less than B), and base is B.
And the answer is
$$\frac{45-20}{45}.100\% \approx 55.55 \% \approx 56 \% $$