**A graphical method for finding when the center is 140 degrees.**

To find the time for the center -- u(t, 0, 3/4) -- to be 140, we could draw make a graphical approximation.

`> `
**plot([140,u(t,0,.75)+550],t=12..13);**

It seems clear that 140 degrees is achieved between 12.9 and 13 minutes.

`> `
**plot([140,u(t,0,.75)+550],t=12.9..13);**

Now, I surmise that the desired t solution is between 12.96 and 12.98.

`> `
**plot([140,u(t,0,.75)+550],t=12.96..12.98);**

`> `

That defines the value of t pretty closely.