Well, first of all, you have to compare the two statements
1 fence= $25
X= $165
The X represents the unknown number of fences which you have to find.
The next step is to cross and multiply which means 165 will multiply 1 and X will multiply 25
Therefore:
165 x 1= X x 25
165=25X
Next, divide both sides by 25
165/25=25X/25
165 divided by 25 is 6.6 which is approximately 7
Hope I helped. Good luck