Final answer:
In the proportion 1:4 :: 3:12, the extremes are 1 and 12, while the means are 4 and 3, conveying the equality of two ratios.
Step-by-step explanation:
In the proportion 1:4 :: 3:12, the terms called extremes are the first and the last numbers, in this case, 1 and 12. The terms in the middle positions are called the means, which are 4 and 3 in this proportion. A proportion indicates that two ratios are equivalent, meaning the product of the extremes is equal to the product of the means (1 x 12 = 4 x 3).