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
