If y varies directly with x, and If y = -16 when x = 4, how do you find x when y = 20?

1 Answer
Mar 2, 2016

When #y =20# , #x# is #color(Blue)(-5#

Explanation:

#y# varies directly with #x# this implies #y# is directly proportional to #x#:
#color(blue)(y prop x#

Upon introducing constant #k# (constant of proportionality ):
#y = color(green)(k) x#........#(1)#

As per given data: when #y=-16, x=4#, substituting the above values in #(1)# to find #color(green)(k#

#-16 = k xx 4#

# k = -16/ 4#

# color(green)(k = -4#,

Finding #x# when #color(blue)(y =20# :

#y=kx#

#color(blue)(20) = color(green)(-4) xx x#

# x = 20/ (-4)#

#color(blue)(x= -5#