DDT is made from chlorobenzene and trichloroethanal (chloral) in the presence of concentrated…

Step 1. Amounts. Chlorobenzene: $\dfrac{25.0\ \text{g}}{112.5\ \text{g/mol}} = 0.222\ \text{mol}$. Chloral: $\dfrac{14.75\ \text{g}}{147.5\ \text{g/mol}} = 0.100\ \text{mol}$.

Step 2. Limiting reagent. The chlorobenzene allows $\dfrac{0.222}{2} = 0.111\ \text{mol}$ of DDT. The chloral allows $0.100\ \text{mol}$. Chloral limits, at $0.100\ \text{mol}$ of DDT.

Step 3. DDT. At full yield: $0.100 \times 354.5 = 35.45\ \text{g}$. At $60\ \%$: $35.45 \times 0.60 = 21.3\ \text{g}$.

Step 4. Chlorobenzene remaining. When all the chloral has reacted, $2 \times 0.100 = 0.200\ \text{mol}$ of chlorobenzene has been used, which is $22.5\ \text{g}$. So $25.0 - 22.5 = 2.50\ \text{g}$ remains.

Step 5. Chlorine in DDT. $\dfrac{5 \times 35.5}{354.5} \times 100 = \dfrac{177.5}{354.5} \times 100 = 50.1\ \%$.