12-7 = 5cm

5:10 = 0.5cm.

8x0.5 = 4cm.

12+4= 16 (cm)

Answer: It turned out to be 16 cm long.

Equation: 1: let the candle be x cm per minute.

12+8x=7+ 18x

12+8x):(7+ 18x)= 1

x=0.5

12+8x= 16

2. Set the candle to be x cm long.

(X- 12):8=(X-7): 18

X= 16

Answer: It turned out to be 16 cm long.