Our favorite program runs in $10$ seconds on computer $A,$ which has a $400MHz$ clock. We are trying to help a computer designer build a new machine $B,$ that will run this program in $6$ seconds. The designer can use new $($or perhaps more expensive$)$ technology to substantially increase the clock rate but has informed us that this increase will affect the rest of the CPU design, causing machine B to require $1\mathrm{.}2$ times as many clock cycles as machine A for the same program. What clock rate should we tell the designer to target?"