#### Answer

Equation: y=2x
y=20 when x=10

#### Work Step by Step

Since we know that y-varies directly with x, we can plug in the values for y and x and solve for k
8=k(4) (Divide both sides by 4)
2=k
So plug k in for the general form of a direct variation (y=kx):
y=2x
Now in order to find the value of y when x=10, plug this value of x into the equation above:
y=2(10)
y=20