 and in this video we're going to look at percentage changes. We have to calculate percentage changes throughout life, whenever we're comparing two values, the increase or decrease in the value of stocks, comparing rainfall or looking at unemployment figures. This is a really simple way of calculating percentage changes. The new value is the chronologically more recent value, the older value and then the multiplier, which is a decimal number. If you cannot remember how to find multipliers, watch this video first. So let's have a look at some examples. This question 2005 is the old value, 2015 is the new value. We want to find the percentage change, so this is the multiplier. Substitute in the values. We don't know the multiplier, so rearrange and solve for m. m equals 0.367. You should remember that multipliers that start as not point something are percentage decreases. And for percentage decreases, we need to subtract 36.7 from 100. There has been a 63.3% decrease in the number of HIV related deaths since 2005. Simple really. You need to know about multipliers and then the formula new equals old times multiplier. Just take your time to work out which is the new value, which is the old value and what the multiplier is. So let's have a look at another example. Work out the new, old and multiplier. The normal price is older, so it's the old value. The sale price is more recent, so it's the new value. The multiplier is 0.8, because it's a 20% decrease. Pause the video, answer the question and click play when you're ready. Did you get it right? Once you get an answer, think back to the question and check if it makes sense. The car was originally 6,500 and then went on sale for 5,200, so that makes sense. Questions like this, where we're asked to find the old or original value, are also known as reverse percentages, but that's not important. All that is important is the new equals old times multiplier formula. Here are some questions for you to do. Pause the video, work them out and click play when you're ready. How did you get on? So there we have percentage changes. You just need to remember this formula. A common thing people can find tricky is working out if it's a percentage change question or if it's just asking us to find a percentage of. If one value isn't obviously older or better than the other, it's a finding a percentage question. So take your time to work out what is being asked and then check your answer makes sense at the end.