Number + 道 + Noun
道 (dào) is a measure word used for long, narrow things such as beams of light, cracks, scars, or lines. It can also be used for doors, walls, and exam questions. For example, 一道光 means "a beam of light."
天空中出现了一道彩虹。
Tiānkōng zhōng chūxiàn le yī dào cǎihóng.
A rainbow appeared in the sky.
墙上有一道很深的裂缝。
Qiáng shàng yǒu yī dào hěn shēn de lièfèng.
There is a deep crack on the wall.
这道数学题太难了,我不会做。
Zhè dào shùxué tí tài nán le, wǒ bù huì zuò.
This math problem is too hard; I can't solve it.