3x-x= 100

2x= 100

X=50 (yuan)

3x = 3x50 = 150 (yuan)

Answer: coat 150 yuan, pants 50 yuan.

Because considering that the third grade of primary school may set an unknown number X for learning, another solution is needed.

Because the coat is three times more expensive than the trousers, the coat is twice as expensive as the trousers. And because the coat is more expensive than the trousers 100 yuan, so the trousers are twice as expensive as 100 yuan, so the trousers are 50 yuan, and the coat 150 yuan.

Coat = pants ×3①

Coat-trousers = 100②

Substitute ① into ② to get: pants× 2 =100.

Pants =50 yuan ③

Substituting ③ into ① gives coat =50×3= 150 (yuan).

Answer: coat 150 yuan, pants 50 yuan.