Use F=ma
The key word is maximum, so you can use values 3ms-2 (acceleration) and 12000N
Let X equal the number of people
Using f=ma
12000N-(300g+75g[x])=(300+75x)3
And find X by making it the subject.
(12000-300g-75gx)/3 = 300+75x
(12000-300g-75gx)/3 - 300 - 75x = 0
(12000-300g)/3 - (75gx)/3 - 300 - 75x = 0
Multiply by 3, and switch sides around.
225x + 75gx = 12000 - 300g - 900
X(225+75g) = " "
X = (12000 - 300g - 900) / (225+75g)
And when rounding, round downwards to the nearest whole number.
So I got 8 people, I hope that is correct!