Inflation has risen in three of the last four months and is slightly higher than it was a year ago. Yet you wouldn’t know it from listening to President Donald Trump or even some of the inflation fighters at the Federal Reserve who have dismissed or even downplayed inflation.
30-to-20 is negative acceleration (2nd derivative) causing a reduction in speed (first derivative) from 30 to 20.
Similarly, inflation is a rate (like a velocity). A constant 10% inflation means prices rise at the rate of 10% per year.