Question

how to find third proportion

Answer 1

3RDS = first proportion ×last proportion by the middle one

Answer 2

Let c be the third proportion of two numbers a & b.

If, a=1 & b=2

Then,
a:b = b:c
1:2 = 2:c
1*c = 2*2 (by cross-multiplication)
c = 4/1
c = 4 Ans.

In this way, you can find third proportion of two numbers.