1 1/4*30=37.5 because, first to make this easier to work with I like to turn the fraction into a decimal like so, 1/4=.25 because 1/4=.25 so 1/4=.25, so now we have 1.25*30=? What this is pretty much saying is that 30+(25% of 30)=? Which to solve this we just need to find how much 25% is of 30 which is .25*30=7.5 because 7.5*4=30 making it true that this is 25% of 30. Now what we have to do is take 30+7.5 (because we have a 1 infront of the 25% we add this instead of subtract it, if we had no whole number infront of the 30 and where solving this that way we would have to subtract instead of add) which adds up to be are end answer of 37.5.
Hope that this helps and makes sense, Enjoy!=)