Answer:
18 but look at others awnsers aswell
Step-by-step explanation:
The given equation is: 
To find the line perpendicular to it, we interchange coefficients and switch the signs of one coefficient.
The equation to a line perpendicular to it is:
$ 2y-x=c$
where, $c$ is some constant we have determine using the condition given.
It passes through $(2,-1)$
Put the point in our equation:
$2(-1)-(2)=c$
$c=-2-2$
$c=-4$
The final equation is:
$\boxed{ 2y-x=-4}$
Step-by-step explanation:
let both unknowns be x and y
x = 5y. ......(1)
x + y = 90. ......(2)
sub eqn (1) into eqn (2)
5y + y = 90
6y = 90
y = 90/6
= 15
sub y = 15 to eqn (1)
x = 5y
x = 5 × 15
× = 75