The mathematical explanation is that if f(x) = x + 2 and y (x) = x -2, the relationship is inverse. Also, f(x) = -x and f(x) = 1/x to eliminate a zero value.
I will send you a hopefully helpful link. https://www.ncdc.noaa.gov/sotc/national/201403
The probability would be 6/16 or 37.5% because there are 16 coins in total and 6 of them are worth more than 5 cents. :)