Statistics: the mean

This is from jan 2006 s1 ocr: 2iii) calculate the mean height of all 150 plants.
We are given the information, 50 plants has mean of 124.5cm, 100 plants has a mean of 123.0cm.
I don't understand what the mark scheme is saying: (124.5+123*2)/3.
Please help!!!

This is from jan 2006 s1 ocr: 2iii) calculate the mean height of all 150 plants.
We are given the information, 50 plants has mean of 124.5cm, 100 plants has a mean of 123.0cm.
I don't understand what the mark scheme is saying: (124.5+123*2)/3.
Please help!!!

I'm not sure about the markscheme, but here's what my solution would be:

Mean = total/n. If the mean for 50 plants is 124.5 cm, then the total height of all those plants is 50x124.5 = 6225cm. The same for the 100 plants, 100x123 = 12300cm. Add these together, 6225 + 12300 = 18525. This is the total height of all 150 plants, so the mean height of all 150 plants is 18525/150 = 123.5cm

Thank you! That method is on the mark scheme too, I understand it but I want to understand the method the mark scheme is saying, as the method you guys are suggesting is only in the 'acceptable' box. But thank you anyway!