In a Jeep there are 3 seats in front and 3 in back. Number of different ways can Six persons of different heights be seated in the Jeep so that every one in front is shorter than the person directly behind is 
 

$$\begin{gathered} \text{ Select one seat at back.It can be done in }{3}_{C_1}\text{ ways,} \\ \mathrm{n}\text{ow tallest person will occupy this seat } \end{gathered}$$

$\text{ and infront of him any one of }5\text{ persons can be seated in }{5}_{C_1}\text{ ways }$

$\text{ Again select one seat and repeat process }{3}_{C_1}\times 5_{C_1}\times 2_{C_1}\times 3_{C_1}=90$