(x + 7): 6 = 7:21 is equal to how much? Half: two seventh = one third: (4-x) is equal to how much? All need formula! On the evening of April 19th, I will answer,

（x+7）：6=7：21
So x + 7 = 6 * 7 / 21
=2
x=-5
(1/2):(1/7)=(1/3):(4-x)
(4-x)/2=1/21
4-x=2/21
x=4-2/21
X = 3 and 19 / 21