In the case of percentage change the TOTAL is the original quantity and the PART is the actual change difference. The increase is 25. It is used for many purposes in finance often to represent the price change of a security.
So in the case of percent of change the generic formula part total becomes.
When finding the percent increase we take the absolute value of the difference and divide it by the original value. This process is the reverse of what you did earlier. The general percentage formula for one quantity in terms of another is multiplying the ratio of the two quantities by 100. 100 30 discount 70.