Question

What is the fourth proportional in the following: 3, 10, 21, ?

Answer 1

Let the fourth proportional (x) 3:10=2l:x X= (21×10)÷3 =7o  Then the fourth propotional is 70

Answer 2

Answer:

The fourth proportional be, 70

Explanation:

If a, b, c ,d be 4 numbers, such that  a : b = c : d  then, d is called the fourth proportional.

Given: 3, 10 , 21 , ?

Let the fourth proportional to be x;

3 : 10 :: 21 : x

The product of the means is the product of the extremes, then we would get,

$10 \times 21 = 3 \times x$

Solving for x:

$x = \frac{10\times21}{3} = 10 \times 7 = 70$

Therefore, the fourth proportional is 70.