 Okay so your task for this video is sort of a thinking problem but you can solve it using the same calculations that I've been using here. You've run 90 litres of cold water with temperature of 20 degrees Celsius into your bath but you find that that's too cold. Question is what volume of water from the hot tap at 60 degrees Celsius would you need to add to get your bath water to 40 degrees Celsius? Just eyeballing that I haven't done a good job with sig figs here. Don't worry too much about sig figs in this case. There is a quick way of doing this rather than having to go through the whole calculation but it may not leap out at you at this point. If it doesn't go through and do the calculations in just the way that we've just been doing and perhaps afterwards have a look at your answer think about it a little bit and see if you can come up with a quicker way of working it out.