Question

One fifth of a number plus five times that number is equal to seven times the number less 18. find the number.

Answer 1

OK.  First we have to figure out names for all the numbers you mentioned.

The mysterious number . . . . . call it Q
One fifth of the number . . . . . 0.2 Q
Five times the number . . . . . . 5 Q
Seven times the number . . . . 7 Q

(One fifth plus five times the number) . . . 5.2 Q
(Seven times the number less 18) . . . . . . 7 Q - 18

You said that these last two things are equal, 

so I can write . . . . . . . . . . . . . . . . . .  7 Q - 18  =  5.2 Q

Now, subtract  7Q  from each side . . .  -18 = -1.8 Q

Divide each side by  -1.8 . . . . . . . . . .  10 = Q