You may have found the correct value of k, but then instead of multiplying the product of 25 and 16 by $\displaystyle\frac{{1}}{{2}}\usepackage{cmbright}\Huge\$ , you may have multiplied the product by 2. The correct calculation would be: $\displaystyle{y}={\left(\frac{{1}}{{2}}\right)}{\left({25}\right)}{\left({16}\right)}\usepackage{cmbright}\Huge\$