Number line refers to a mathematical process of
solving the equation with the use of lines.
=> 235 + 123
Starting from 0 you count 1 up to 235. Then starting from 235 you additionally
count another 123.
Then from zero start counting the total number to the line you stopped when
adding 123 to 235.
This simply equals to
=> 235 + 123
=> 358.
Pls. see attached image for illustration of number line.Answer here
