Let x= ones digit
x+3= tens digit
Equation:
x+x+3=13
combine them
2x+3=13
2x+3-3=13-3
2x=10
2x ÷ 2=10÷2
x=5
x+3=8
the answer is 85
8+5=13
To understand algebraic expression...
whatever you do in the left side or right side... you must do it on the other side
The answers can be negative!! IT DEPENDS
Don't worry just trust me :P and when you're grade 6/7 this will be tough on you as soon as possible XD just trust me ;)
You can do gues and check lol (just realize it after a couple of minutes )
Guess of two one-digit numbers that their sum is 13
9,4
8,5
7,6
then one number must be 3 greater than the other which is 8 and 5
SO... 85