Imagine you have a classmate Peter, and it is rather hard to observe his height with bare eyes. But when you are standing right next to him, you know you are approximately 5cm shorter than him.
Because knowing you are 130cm tall, by using algebra, you can come up with an algebraic equation of 130 + x, in which letter x in this case, represents the 5cm difference. By substituting 5 to x, you now know Peter is 135cm tall.
Here we give another example. There are two sisters. The big sister’s age is double that of the little sister. We now want to use an algebraic equation to illustrate the relationship of the ages of the two sisters.
Firstly, we have to use some letters as notation for big sister’s and little sister’s ages. Let us use B to stand for bigger sister’s age and L to stand for litter sister’s age.
By know that the big sister’s age is double of the little sister’s, we have B = 2L, or on the contrary, L = B divided by 2.
Riding on the example of sister’s ages, we now have a father. The father is 30 years older than the big sister. Assume the little sister is 4 years old, so what is the father’s age?
Let’s use F to stand for the father’s age. We have F = B + 30 and B = 2L.
We can substitute L by 4 and easily obtain the result of B = 8. Next, by using the above equation of F = B + 30, we can obtain F = 38. Therefore, we know that the father is 38 years old.
To conclude:
Firstly, use some letters to stand for any unknown you want to know. It is better if the letters’ initials are somehow related to the words. For example, F stands for father’s age.
Secondly, illustrate the relationship to equations. For example, F = B + 30 and B = 2L. The relationship should be as simple as possible.
Finally, put all the known values into the equations, and obtain the answers you want to know.