Question 1128884
The key to the answer is finding the slope for the lines.
Solve each equation for y to find the slope-intercept form of each equation.
{{{5x+4y=40}}} --> {{{4y=40-5x}}} --> {{{y=40/4-5x/4}}} --> {{{y=(-5/4)x+10}}}
{{{16x=15+20y}}} --> {{{16x-15=20y}}} --> {{{y=16x/20-15/20}}} --> {{{y=(4/5)x-3/4}}} 

we got different expressions for y, so the lines are not the same line in disguise.
The slopes (the coefficients of x) are {{{-5/4}}} and {{{4/5}}} .
Their product is
{{{(-5/4)(4/5)=-1}}} .
The fact that the product is {{{-1}}} means that the lines are perpendicular.