In order to mark something as ‘equal to, or less than (A)’, 以下（いか） will be used. This is simply a combination of the kanji 以（い） which means ‘by means of’, and 下（か） ‘below’. This expression may also be translated as ‘under (A)’, or ‘fewer than (A)’.
To use 以下（いか）, attach it to the amount that you want to highlight as the ‘maximum’ amount.

• 17歳（さい）以下（いか）の方（かた）は保護者（ほごしゃ）の方（かた）と来（き）てください。
  For those under the age of 17, please come with a guardian.
• この車（くるま）は30万円（まんえん）以下（いか）だった。
  This car was under 300,000 yen.

Similarly to the way that 以上（いじょう） can be used to mark ‘everything up until now’, 以下（いか） can and often is used to highlight ‘everything after now’, ‘the rest’, or ‘the following’.

• 以下（いか）同文（どうぶん）。
  The rest is the same. (Usually seen in documents to notify the reader that the rest of the document basically repeats what has already been said)
• 空港（くうこう）には以下（いか）のものを持（も）って来（き）てください。
  Please bring the following to the airport.