I need help on how to explain to my 5th grader how to come up with an algebra equation from a word problem. Any suggestions?

Question

Example:
Gerry had a total of 30 pens and pencils. He decided to trade with his friends all his pens for pencils. If he traded every pen for 2 pencils he would have 48 pencils in all. How many pens and how many pencils did he have before the trade

Answer 1

Let his original number of pens be x
then his original number of pencils is 30 - x
So for every pen he gets 2 pencils
so x pens will yield him 2x pencils

but he already had 30-x pencils, so

2x + 30-x = 48
x = 18

So, originally he had 18 pens and 12 pencils

check:
12+18 = 30 , good!
for each of the 18 pens he will get 36 pencils
36+12 = 48
My answer is correct.

Answer 2

Mrs. Jackson bought 23 dozen pencils. She gave 15 pencils to her students. How many pencils does she have left?

Answer 3

23 multiplied by 12= 276
276-15= 261
The answer is 261

Answer 4

1 pen = 2 pencils
? pen = 48 pencils
? = 24 since 48/2 = 24

30 - 24 = 6 pencils

48 - 6 = 42 pencils

42/2 = 21 pens

The answer is 6 pencils and 21 pens