Find the percent of increase. round to the nearest tenth of a percent where necessary.

$5.10 to $8.50

The simplest way to do this is, first, to find the difference between the two values.
8.50 - 5.10 = 3.40

Then, just find $3.40 as a percentage of the original value, $5.10, to find the increase.
3.4 / 5.1 = 0.6 recurring ---> Multiply this by 100 to find the percentage.
0.6 recurring x 100 = 66.7% (to the nearest tenth of a percent)

The increase was 66.7%, to the nearest tenth of a percent.