Question

A set of silverware contains 30 pieces. The set contains only spoons, forks, and knives. It contains the same number of spoons and forks and twice as many spoons as knives. How many forks are there in the set?

Answer 1

Let the number of knife be x.
Knife = x
Spoon = 2x
forks = 2x

Total = 30
x + 2x + 2x = 30
5x = 30 
x = 6

Knife = x = 6
forks = 2x = 12
Spoon = 2x = 12

Answer: There are 12 forks.

Answer 2

S = f   (where s = number of  spoons and f = number forks)
s = 2k ( k = number of knives)
s + f + k = 30
2s + k = 30 ( because  s = f)
4k + k = 30
k = 6 
Number of forks (f) =  twice number of  knives = 2*6 = 12 Answer