Finding today's price in the past....
1987 (Base) CPI was 113.485
1979 (Current) CPI was 79.915
The deflation rate between them was (79.915 - 113.485) / 113.485 * 100% = -29.58% inflation
Take $20 worth of cheese in 1987.
In 1979, that cheese would cost $20 + $20 * (-.2958) = $20 (1 - .2958) = $14.08.
I can also do an index factor of 79.915 / 113.485 = .7042. From here, .7042 * $20 = $14.08.
HINT: Want to check your work....
Use your price $14.08 and the index factor moving forward in time....$14.08 * 1.42 = $20.00