Hello there! I can help you! The formula for finding the amount of interest is prt, which means multiply the principal (inital amount) by the rate (simple interest percentage) by the time (could be in months or years)
1. Okay. $328 is the principal, with 5% interest earned annually. 328 * 5% (0.05) is 16.4. That amount was earned over 3 years. 16.4 * 3 is 49.2. There. $49.20 was earned in interest.
2. 635.85 is the principal and was invested at 7% for 6 months. If you know about time, 6 months is equal to ½ a year. ½ is equivalent to 0.5 in decimal form. Let's do the steps. 635.85 * 7% (0.07) is 44.5095 or 44.51 when rounded to the nearest hundredth. Multiply that amount by 0.5 to get the amount of interest earned. 44.51 * 0.5 is 22.255 or 22.26 when rounded to the nearest hundredth. Now, let's add that to the principal. 635.85 + 22.26 is 658.11. The amount in the account is $658.11.
3. Now, we have to find the amount he invested. Let's do the steps in reverse. The formula for finding principal is I / (RT), meaning you divide the interest earned by the rate and time. Rate is always in decimal form and with time being in months, that will be in decimal form as well. Let's start by multiplying rate and time. 9 months is 3 quarters of a year, which is 3/4 of a year. 3/4 is 0.75 in decimal form. 8% (0.08) * 0.75 is 0.06. Interest earned is $110, so now let's divide. 110/0.06 is 1,833.3333333 or 1,833.33 when rounded to the nearest hundredth. The amount of money Tom invested was $1,833.33.
