16, [?] , 4 

let the missing term be x:

, , 

Now use the fact that in a geometric sequence,





Cross multiply:





x = ±8

So that one has two possible answers, +8 and -8.

-----------------

25, [?] , 225

let the missing term be x:

, , 

Now use the fact that in a geometric sequence,





Cross multiply:





x = ±75

So that one has two possible answers, +75 and -75. 

-----

2, [?] , 50 

let the missing term be x:

, , 

Now use the fact that in a geometric sequence,





Cross multiply:





x = ±10

So that one has two possible answers, +10 and -10. 

-----

1, [?] , 49 

let the missing term be x:

, , 

Now use the fact that in a geometric sequence,





Cross multiply:





x = ±7

So that one has two possible answers, +7 and -7. 

-----

3/4, [?] , 3 

let the missing term be x:

, , 

Now use the fact that in a geometric sequence,





Cross multiply:







x = ±

So that one has two possible answers, + and . 

-----

36, [?] , 4

let the missing term be x:

, , 

Now use the fact that in a geometric sequence,





Cross multiply:





x = ±12

So that one has two possible answers, +12 and -12.

Edwin