Review queue

Spaced repetition: each system comes back just before you would forget it. Fast, correct answers stretch the interval; a miss brings it back the same day.

Free practice: 991 System (991 시스템)Score 0/0
01020304001020051015202530123
On the table, the ringed ball is your ball (1). 2 = the ball your cue ball hits first, 3 = the last ball to hit for the point.
991 System (991 시스템)
4th cushion = Start − Aim
Your ball:White(cue ball · 1)
Ball 2YellowBall 3Red
Expected shot:991 System (991 시스템)Bricole (cushion first)Cushion orderShort → Long → Short → Short

The cue ball goes to the cushions before touching any ball; after the cushions in order it hits the balls.

Given:
Aim20Start40
In this position, what is 4th cushion?

Task: put the given values into the system's formula and calculate 4th cushion. Enter the answer as the number read on the table's 4th cushion scale.