Question 760731
Principle 1: Number of cars built is directly proportional to number of engineers. (i.e. more engineers, more cars built)

Principle 2: Time taken to build cars is inversely proportional to number of engineers (i.e. if you have more engineers, it  will take a lesser time to build the same number of cars)

Principle 3: Number of cars is directly proportional to the time taken. (i.e. if you have more cars to build, it will take longer - if the number of engineers is the same)


Now, given that 12 engineers make 30 cars in 6 days.

i.e. 1 engineer can make 30 cars in {{{6 * 12 = 72 }}}  days
---> because time taken is inversely proportional to number of engineers 

i.e. 1 engineer can make 1 car in {{{72 / 30 = 2.4}}}  days


Now if we have to build 15 cars with 18 engineers?

1 engineer can make 15 cars in {{{2.4 * 15 = 36 }}}  days

So, 18 engineers can make 15 cars in {{{36 / 18 = 2 }}} days

Ans: 18 engineers can make 15 cars in 2 days.

:)