The constant of proportionality for y=5x is 5. In a proportional relationship of the form y=kx, the constant of proportionality k is the fixed ratio between y and x—it tells you how much y changes for every unit change in x. In this equation, for every increase of 1 in x, y increases by 5. You can verify this by testing values: when x=1, y=5; when x=2, y=10; when x=3, y=15. The ratio y/x always equals 5. The constant of proportionality represents the slope of the line when graphed on a coordinate plane, and it's also the unit rate in real-world contexts. For example, if y represents distance in miles and x represents time in hours, a constant of proportionality of 5 would mean traveling at 5 miles per hour.