Let's define the next variables:
x: duration of plan A, in hours
y: duration of plan B, in hours
On Friday there were 4 clients who did Plan A and 8 who did Plan B, and the total time of training was 9 hours, then:
4x + 8y = 9 (eq. 1)
On Saturday there were 2 clients who did Plan A and 3 did Plan B, and the total time of training was 4 hours, then:
2x + 3y = 4 (eq. 2)
Multiplying equation 2 by 2, we get:
2(2x + 3y) = 2*4
2*2x + 2*3y = 8
4x + 6y = 8 (eq. 3)
Subtracting equation 3 to equation 1:
4x + 8y = 9
-
4x + 6y = 8
-------------------
2y = 1
y = 1/2
Substituting this result into equation 1 and solving for x:
4x + 8*1/2 = 9
4x + 4 = 9
4x = 9 - 4
4x = 5
x = 5/4
In conclusion, plan A lasts 5/4 hours, and plan B lasts 1/2 hours