A demand paging system,with page table held in integer,takes $5ms$ to service a page fault if an empty page is available,or if the page to be replaced is not dirty.It takes $15ms$ if the replaced page is dirty. Memory access time is $1\mu s.$ Assume we want an effective access time of $2\mu s$ and that the page to be replaced is dirty $60\%$ of the time. The approximate maximum acceptable page fault rate to meet this time requirement will be______$\%?$
$\text{(Correct to two decimal places).}$