Hi
to solve you must transcrive every word in math terms correctly.
so let's first give a name to our mysterious number. As you know, in math we generally call unknown by letter. " X"
so let's now play the detective.
what do we know on " X" ?
we add 3 to X so we have : X+3
then we multiply by 5. so. ( X+3) *5 .
if we do that we have twice X , so 2X
so ( X+3) * 5 = 2 X
now that we have translatted our data in math term, let's unmask. " X"
( X+3) *5 = 2X
5*X +3*5 = 2X
5X +15 = 2X
5X-2X = -15
3X = -15
X = -15/3
X = -5
So is " -5" is our guy.
let's check it out to be sure .
-5 +3 = -2
-2*5 = -10
-10 is twice -5. so -5 is the correct answer.
Ps : some operation are commonly implied. if you do not. know how I could write some line in the calculus, dont hesitate to ask in comments.
There is no mysteries in math everything is done according to rules and can be explained. So don't hesitate !