Larry S. answered • 04/29/20
Cornell Engineering Grad for SAT Math, Physics, all High School Math!
There's a quick way to do this kind of problem. Quick, simple, no equations, but it takes practice to be good at it!
We want to find how much money Marty invested in bonds, so let's start by imagining that Marty invested ALL of the money in the other option, stocks.
He would have made $450 in interest ($9,000 * .05). This is $75 short of what we want ($525 - $450), so we have to take some of those dollars and invest in bonds instead.
How many dollars? Well, each time he changes a dollar from investing in stocks to investing in bonds, he makes $.03 ($.08 - $.05), so he has to do it 75/.03 times. That's (75/3)(100)--- $2500 in bonds.
This may seem mysterious at first, but you can get really good at it. I teach this method to all of my SAT Math students, because there are a lot of these "2 product" problems on the SAT!
