One day a store sold 30 sweatshirts. white ones cost $11.95 and yellow ones cost$11.50. in all, $349.95 worth of sweatshirts were sold. how many of each color were sold?

With this, I would use a guess and check method.
Start by splitting them half.
$11.95x15 sweatshirts = $179.25
$11.50x15 sweatshirts = $172.50
Then you add the totals to see how close you are.
$197.25+$172.50=$351.75
That's a little high. so we'll shift the amount of sweatshirts.
$11.95x14 sweatshirts= $167.30
$11.50x16 sweatshirts= $184.00
And add the totals again.
$167.30+184.00= $351.30
Closer, but too high still, so we'll shift it more in the direction we're going.
$11.95x11 sweatshirts = $131.45
$11.50x 19 sweatshirts = $218.50
Once again, add the totals to check if it matches.
$131.45+$218.50= $349.95
Therefore, 11 white sweatshirts and 19 yellow sweatshirts were sold.