Question

half the sum of x and y decreased by one-third y

Answer 1

$half\to\frac{1}{2}\\the\ sum\ of\ x\ and\ y\to x+y\\decreased\to"-"\\one-third\ y\to\frac{1}{3}y\\\\an\ expression:\\\\\frac{1}{2}(x+y)-\frac{1}{3}y$