Question

What is fourth proportional to 3,9,21?
A- 63
B- 27
C- 81
D- 54

Answer 1

This is a geometric sequence. We can divide one of the values by the one before it.

i.e. 21 and 9.

21/9 = 3

Each term is increasing by a factor of 3.

3*3 = 9
9*3 = 21

So what is 21*3?

Answer 2

9*3=27

Answer 3

Hm.

Answer 4

Does the question look like this?

3;9 = 21;x

Answer 5

Or just like

3;9;21;x

Answer 6

3,9,21,? like this

Answer 7

Nevermind. We just "cross-multiply"

3;9;21;x

3x = 21(9)

3x = 189
3x/3 = 189/3
x = ?

Answer 8

Yeah I thought this was a geometric sequence, I was wrong. Hehe. ^That should be the right way to do it even though I came out with the same thing.

Answer 9

i think
3*2=6
6+3=9
second is
9*2=18
18+3=21
tried is not their options
21*2=42
42+3=45

Answer 10

45 is not right

Answer 11

Where did you get all that from?

Answer 12

in ps exam books?

Answer 13

I know this doesn't show up in higher level math , so that's a good sign

Answer 14

which one is right option