# Aptitude - Chain Rule - Discussion

Discussion Forum : Chain Rule - General Questions (Q.No. 11)
11.
A flagstaff 17.5 m high casts a shadow of length 40.25 m. The height of the building, which casts a shadow of length 28.75 m under similar conditions will be:
10 m
12.5 m
17.5 m
21.25 m
Explanation:

Let the height of the building x metres.

Less lengthy shadow, Less in the height (Direct Proportion)

40.25 : 28.75 :: 17.5 : x       40.25 x x = 28.75 x 17.5

 x = 28.75 x 17.5 40.25

x = 12.5

Discussion:
12 comments Page 1 of 2.

Nima Tenzin said:   1 year ago

ABI said:   3 years ago
Thanks for the good method @Feyi.

Feyi said:   7 years ago
Let the height be 'a' and length of shadow be 'b'.

The higher the height, the longer the shadow (ie height is directly proportioned to the length of the shadow).

The relationship is a & b.

a = kb (k=proprtionality constant).
k = a/b.
=17.5/40.25.

The question is when b =28.75 what is a?

a = kb.
= (17.5/40.25) * 28.75.
=12.5.

Therefore, the height of the building is 12.5m.

Thanks.

Shiva said:   7 years ago
How can you say shadows height is directly proportional to the height of the building?

Anonymous said:   7 years ago
Chain rule for solving the solution is :

Direct proportion means first what we have then what we want.

Indirect proportion means first what we want next what we have.

x always in the second field.

Multiply x with first values equals to multiply with second values.

Jayesh said:   7 years ago
Here is the proper solution:

17.5m height can produce 40.25 m length shadow.

So, now it is asked that 28.5m length shadow of what height.

Simply cross multiply,

Height <-> Length.
17.5 <-> 40.25.
? <-> 28.75.

=> 28.75 x 17.5/40.25 = 12.5m -->height.
(1)

Balaji said:   8 years ago
Please explain me briefly, I can't understand the question.

Ganesh said:   10 years ago
So that considered round figure.
40250/175 = 230.

Which casts a shadow of length 28.75m.
Considered round figure = 28750/230 = 125.
So The height of the building is 12.5.