adapt columns to page width

Hi!
I have got 7 columns in my page. I wrote on the code: ptp-seven-col { width: 23%;} so that all are the same width, but obviously the last 3 columns go beyond the page width, they should be placed below the first 4. How can I do that? how can I wirte int he code that one the max width is reached, the next columns have to be placed below?
thank you

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