You are given a hammer and a nail that costs $31. You are also given the hammer which costs $30 dollars more than the nail. You are asked to find the cost of the nail. Let x be the the cost of the cost of the nail.
The phrase "$30 dollars more than the nail" is equal to 30 + x
Also, a hammer and a nail that costs $31. Combining the two, we will have,
30 + x = 31
x = 31 - 30
x = $1
The cost of the nail is just $1.