Easy ratio maths! help

1. If x:3 = 12 : x, calculate the positive value of x(how do i do this sort of maths)
this means x/3 = 12/x
cross multiplying
x*x=12*3 =36
x^2= 36 = (+6 or -6 )^2
taking square roots and choosing positive values
x =6
2.If y:18= 8:y, calculate the positive value of y.
this means y/18 = 8/y
cross multiplying
y*y=18*8 =144
y^2= 144 = (+12 or -12)^2
taking square roots and choosing positive values
y = 12

1. x:3 = 12:x

Those terms can be rewritten as fractions:
x/3 = 12/x

Cross-multiply:
3*12 = x^2
36 = x^2
Square root both sides; x = 6 or -6. They ask for the positive answer, so x = 6.

2. Rewrite the terms as fractions:
y/18 = 8/y

Cross-multiply:
18*8 = y^2
144 = y^2

Square root both sides; the positive answer is y = 12.

3. Let f represent the amount of flour, s represent the amount of sugar, and r represent the amount of raisins used. From the problem, we know that
f + s + r = 550
f = 2s
s = 3r/2; multiplying both sides by 2/3, 2s/3 = r

Plug each of the last two equations into the first and solve for s:
2s + s + 2s/3 = 550
11s/3 = 550
s = 150; there are 150 grams of sugar.

f = 2s, so there are 300 grams of flour.
r = 2s/3, so there are 100 grams of raisins.

1. convert these ratios into division.
Ex. : x : 3 = 12 : x => x/3 = 12/x
then, finish the equation.
x/3 = 12/x
x * x = 12 * 3
x^2 = 36
x = square root 36
x = 6

2. the same way like no.1
y/18 = 8/y
y * y = 18 * 8
y^2 = 144
y = square root 144
y = 12

3. abbreviate flour as F, sugar as S, raisins as R.