4 / 21 spoonful of vanilla
Explanation:
3 1/2 dozens = 42 cookies
Let x = spoonful of vanilla for 1 dozen of cookies
2/3 : 42 cookies
x : 12 cookies
2/3 ÷ 42 = x ÷ 12
2/3 × 1/42 = x / 12
2 / 126 = x / 12
Cross product
2*12 = 126*x
24 = 126x
Divide both sides by 126
x = 24 / 126
= 4 / 21
x = 4 / 21 spoonful of vanilla
Per dozen of cookies requires 4/21 spoonful of vanilla
Check:
4/21 per dozen × 3 1/2 dozens
4/21 × 7/2
28 / 42
2/3 spoonful for 3 1/2 dozens