Question

The fourth proportional to 3, 5 and 21 is​

Answer 1

Let the no. be x

3:5::21:x

= 3:5 = 21:x

cross multiply

= 21x5 = 3x

= 105 = 3x

= x = 35

Hence the fourth proportional is 35.

Answer 2

The fourth proportional number is 35.

Given: The numbers in proportion 3, 5, and 21.

To Find: The fourth proportional.

Solution:

We shall solve this numerical by the concepts of proportionality.

• According to the concept of proportionality, if there are four numbers given to us in proportion, then the ratio of the first and second numbers will be equal to the ratio of the third and fourth numbers.

Coming to the numerical, we are given;

The numbers in proportion = 3, 5 and 21

Let the fourth number be 'x'.

As said earlier, we can say that;

             3: 5 :: 21: x

         ⇒ 3 / 5 = 21 / x

         ⇒   x  = ( 21 × 5 ) / 3

         ⇒   x  =  7 × 5

                   = 35

Hence, the fourth proportional number is 35.

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