Number/Noun + 以下
以下 means "below," "less than," or "and under," placed after a number or noun to indicate everything at or beneath that threshold. It is commonly used with quantities, ages, and levels. For example, 零度以下 means "below zero degrees."
三岁以下的儿童可以免费入场。
Sān suì yǐxià de értóng kěyǐ miǎnfèi rùchǎng.
Children under three years old can enter for free.
请将室温保持在二十五度以下。
Qǐng jiāng shìwēn bǎochí zài èrshíwǔ dù yǐxià.
Please keep the room temperature below twenty-five degrees.
五百元以下的东西不需要审批。
Wǔbǎi yuán yǐxià de dōngxi bù xūyào shěnpī.
Items under five hundred yuan don't require approval.