A ruler has only four scales (1, 4, 5, 11), but it can measure all the whole centimeter length of 1-13cm in two places. How to use this ruler, Measure the length of 1-13 cm?

A ruler has only four scales (1, 4, 5, 11), but it can measure all the whole centimeter length of 1-13cm in two places. How to use this ruler, Measure the length of 1-13 cm?

1 cm = 4-5;
2 cm = 4-1 minus 4-5
3cm = 4-1
4cm = 1-5cm
5cm = 1-5 plus 4-5
6 cm = 5-11
7 cm = 4-11
8 cm = 4-11 plus 4-5
9 cm = 1-4 plus 5-11
10 cm = 1-11
11 cm = 1-5 plus 4-11
12 cm = 5-11 measure twice
13 cm = 1-4 plus 1-11

An incomplete ruler has scales only at "1cm", "4cm" and "9cm". The length that can be measured with this ruler at one time is
What are the centimetres?

Six lengths can be measured at one time, they are 1cm, 3cm, 4cm, 5cm, 8cm and 9cm

Xiaohua accidentally broke the ruler. The smallest scale is 5cm and the largest scale is 20cm. Now how much can she measure at most at one time
How many centimeter long objects can you measure at most?

How many centimeters can you measure
= 20 - 5 + 1
=16 cm