Multiply both sides by #5#, then divide both sides by #4#.
# 4/5z=-20 #
# 4/5zcolor(blue)(*5)=-20color(blue)(*5) #
# 4/color(red)cancelcolor(black)5zcolor(blue)*color(red)cancelcolor(blue)5=-20color(blue)(*5) #
# 4z=-20color(blue)(*5) #
# 4z=-100 #
# color(blue)(color(black)(4z)/4)=color(blue)((color(black)(-100))/4) #
# color(blue)(color(black)(color(red)cancelcolor(black)4z)/color(red)cancelcolor(black)4)=color(blue)((color(black)(-100))/4) #
# z=color(blue)((color(black)(-100))/4) #
# z=-25 #