1. Explain it simply
To say one amount is a whole-number multiple of another, use A 是 B 的 N 倍 - 'A is N times B.'
Put the number right before 倍: 三倍 (three times), 两倍 (twice).
Keep both 的 and 倍: 的 links the baseline B to the number, and 倍 means 'times.'
The thing you are measuring (A) comes first; the baseline (B) sits between 是 and 的.
2. Show examples
这个学校的学生是那个学校的三倍。
Zhège xuéxiào de xuésheng shì nàge xuéxiào de sān bèi.
This school has three times as many students as that one.
A 是 B 的 三 倍 - three times the baseline.
他的体重是儿子的两倍。
Tā de tǐzhòng shì érzi de liǎng bèi.
His weight is twice his son's.
两倍 = twice; the number sits right before 倍.
今年的游客是去年的三倍。
Jīnnián de yóukè shì qùnián de sān bèi.
This year's visitors are three times last year's.
去年 is the baseline B, sitting between 是 and 的.
3. Share a tip
Drop either 的 or 倍 and the multiple falls apart - always say 是 B 的 N 倍, with 倍 right after the number.