You have $55,000 in your savings account that pays 2.5% annual interest and the inflation rate is 3.4%. how much buying power in dollars will you lose in one year because of inflation?

Your interest formula is given to you. 

Interest in a year = principal (the amount invested) * rate (the interest rate) * period (the time you're measuring) 

Interest = 55,000 * 2% * 1 year = 55,000 * 0.02 * 1 = $1,100 

How much would you need to have made for your spending power to keep with inflation? Your interest rate would have needed to match the inflation rate, otherwise, prices are going up faster than you're saving. 

Required interest = 55,000 * 3.24% * 1 year = 55,000 * 0.0324 * 1 = $1,782 

How much buying power did you lose? The difference between your required interest and your actual interest. 
Buying power lost = 1,782 - 1,100 = $682. You lost this much in buying power.