Answer:
180 cm
Step-by-step explanation:
You want the least common multiple of 20 cm and 45 cm.
The greatest common factor of 20 and 45 is 5:
20 = 4·5
45 = 9·5
The least common multiple of 20 and 45 is ...
LCM = 20 · 45 / GCF
LCM = 20 · 45 / 5 = 180
The stacks of boxes will be the same height when both stacks are 180 cm. There will be 9 boxes in the 20 cm stack, and 4 boxes in the 45 cm stack.