WRED default values?

HI,

I'm doing a lab on WRED.

The question asks to configure 3 class maps "Priority" (35%), "Bulk"(25%)
and "Best-effort" (20%) with precedence assigned, 4, (2 and 3), and (0 and
1). THe WRED drop probability it says is "AS PER DEFAULT FOR THE PRECEDENCE
VALUE/S"

(being configured on an FE interface)

I'm a bit confused with how the solution determines the default precedence
value. For example, the solution says:

for class Best-effort:
R1(config-pmap-c)#random-detect
R1(config-pmap-c)#random-detect precedence 0 *20 40* 10
R1(config-pmap-c)#random-detect precedence 1 *22 40* 10

for class Bulk:

R1(config-pmap-c)#class Bulk
R1(config-pmap-c)#random-detect
R1(config-pmap-c)#random-detect precedence 2 *24 40* 10
R1(config-pmap-c)#random-detect precedence 3 *24 40* 10

for class Priority:

R1(config-pmap-c)#class PRIORITY
R1(config-pmap-c)#random-detect
R1(config-pmap-c)#random-detect precedence 4 *28 40* 10

I can't figure out how the bolded values were determined for this solution.
Any link that explains this would be appreciated....I couldn't really find a
good explanation in the doc cd for this:
http://www.cisco.com/en/US/docs/ios/qos/configuration/guide/config_wred_ps6350_TSD_Products_Configuration_Guide_Chapter.html#wp1006740
http://www.cisco.com/en/US/docs/ios/qos/configuration/guide/congestion_avoidance_ps6350_TSD_Products_Configuration_Guide_Chapter.html

thanks

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