CHAIN RULE

Direct Proportion:-  Two quantities are said to vary directly if on the increase (or decrease) of the one, the other increases ( or decreases) to the same extend.
For eg.   If 5 balls cost Rs 8, what do 15 balls cost?
              Here, if the number of balls increased,
                        then the price will also be increased
                $\therefore $   5 balls : 15 balls : : Rs 8 : required cost
                $\therefore $   the required cost = Rs $\frac{15\times 8}{5}$ = Rs 24.


Indirect Proportion or Inverse Proportion:-  Two quantities are said to vary indirectly, if on the increase (or decrease) of the one, the other decreases (or increases) to the same extend.
For eg.   If 15 men can reap a field in 28 days, in how many days will 10 men reap it?
              Here,  the inverse ratio of the number of men is equal to the ratio of the corresponding number of days.
                 $\therefore $   $\frac{1}{15}$ : $\frac{1}{10}$ :: 28 : required number of days
                 or    10:15::28:required number of days
                 $\therefore $   the required number of days = $\frac{15\times 28}{10}$  = 42 days


Rule of Three:-  The method of finding the 4th term of a proportion when the other three are given is called Simple Proportion or The Rule of Three.