My son has this:
Calculate the fourth proportional of the given numbers: 1/9, 1/6, 1/3
Similarly, another question:
Calculate the fourth proportional of the given numbers: 9.1; 6.5; 2.8
It should be something really simple because he is only in 7th grade, but for some reason I myself don't get it.
Thank you!
Finding the $4$th proportional is an exercise in executing "product of means"="product of extremes"
For example on problem 1, you would solve $$1/9:1/6::1/3:x$$
Since the product of the means is the product of the extremes, you would get $$ 1/6 \times 1/3 = 1/9 \times x$$
Solving for $x$ gives $x=\frac{1}{2}$.
The second example is similar. The correct answer should be $2$. (Obtained by setting $9.1x=(6.5)(2.8)$ )
