In drilling world’s deepest hole, it was found that the temperature T in degree celsius, x km below the surface of the earth, was given by



At what depth will the temperature be between  and ?

We have
                   
It is given that 200 < T < 300
or                                 200 < 30 + 25(x - 3) < 300
or                                  170 < 25 (x - 3) < 270
or                              
or                          6.8 + 3 < x < 10.8 + 3
or                                 9.8 < x < 13.8
 Hence, the depth will be between 9.8 km and 13.8 km.