The investment of "x" dollars gave him a 15% increase that reach to a total of $1250.
Then x + increase = 1250
Now they tell us that the increase was in fact 25% of the value "x" that he invested. So that increse is: 15% of x , wich in math terms is written in decimal form as: 0.15 * x
Then we replace this in the very first equation we wrote:
x + increase = 1250
x + 0.15 x = 1250
combine the terms with "x":
1.15 x = 1250
solve for x by dividing by 1.15 both sides:
x = 1250 / 1.15
Please notice that any of the equations we have written could be considered as correct answers for the problem. Some are more at the beginning of the reasoning, while the last ones are after we have done some algebraic manipulations to solve for the unknown. But they are all equivalent.
If you are given options to select from, check all those that are like the ones we discussed above.