While gathering my thoughts and notes to write my reaction paper to the NCTM standards and Common Core vision I was reminded of percentages. For me, it's embarrassingly difficult to figure out on the fly what a good tip is. I have to sit and stare at the receipt for a good minute or two before the correct number pops into my head. Then I go through it again just to make sure I'm leaving a good tip rather than a lousy one. My kids, however, can come up with the amount in seconds. So this week I worked on sharpening those skills. I can't say I'm a wiz at it yet, but my hesitation is less dramatic and I'm more sure of my answer. One method I found helpful was rounding to the nearest 'easy' number to estimate, as we talked about in class. It doesn't get the perfectly correct number but if I'm rounding up, then at least I know the server won't feel that I left too small of a tip.

Percentages are like fractions. 3% is 3/100 or 3 hundredths. Easy enough. But what about when you need to figure out 17% of  250 strawberries because you are making an exceptionally precise jam? I learned that you can assume that 250 is 100% because it is all of the strawberries. X will be the number of strawberries we need for this amazing jam.

X=17% 250=100

If we turn this into fractions it would be  250/x=100/17%. Things from here got a little rusty so I researched how to make this work and found this explanation:

250/x=100/17(250/x)*x=(100/17)*x       - we multiply both sides of the equation by x250=5.88235294118*x       - we divide both sides of the equation by (5.88235294118) to get x250/5.88235294118=x 42.5=x x=42.5

17% of 250=42.5

I'm going to have to play with this a bit more. But I'm happy to know that we need 42 and a half strawberries to make a batch of jam. Now, how much sugar and lemon juice do we need......?

Math joke of the week:

Q: Why don't you do arithmetic in the jungle? 
A: Because if you add 4+4 you get ate!