Quant English Quirks

For non natives how find it hard to understand the meaning of some english keywords that describes the relations between 2 qunataties

1. A number of girls joined the class just as twice that number of boys left the class This means the number boys left is Twice the number of girls joined means the number of boys who left is twice the number of girls who joined. The wording just as twice that number refers back to the number of girls. Let the number of girls who joined be xxx. Then:

Girls\ joined = x , Boys\ left = 2x