x = 4
Every step shown. Once you become used to doing this you will almost be able to do the basic one's in your head without writing much down.
Explanation:
Let the unknown value be
x
Converting the words into numbers:
First part: "15 times a certain number" → 15 x
Second part: "plus 5 times the same number" → 15 x + 5 x
The last part: " is 80" -> 15x + 5 x = 80
We are counting x ' s . 15 of them plus another 5 of them gives a total of 20.
So 15 x + 5 x = 20 x = 80
Divide both sides by 20
20 x ÷ 20 = 80÷ 20
20/20x=80/20
x=4