Question

double line c. 5 cm 5 cm line c doubled is cm long.

Explanation:

Step1: Identify single - line length

The length of one part of line C is 5 cm.

Step2: Calculate double - line length

Since line C is composed of two equal - length parts each 5 cm long, we use multiplication. The formula is $2\times5$.
$2\times5 = 10$

Answer:

10