Question 950250
x=original members; a=original assessment;
ax=$400
x=$400/a
(x+4)(a-$5)=$400 Set equations equal. 
(x+4)(a-$5)=ax Multiply.
ax+4a-$5x-$20=ax Subtract ax from each side.
4a-$5x=$20 Substitute for x
4a-$5($400/a)=$20 
4a-$2000/a=$20 Subtract 4a from each side
-$2000/a=$20-4a Multiply each side by a
-$2000=$20a-4a^2 Add 2000 to each side
0=-4a^2+20a+2000*[invoke quadratic "a", -4, 20, 2000 ]
a=$25 The original assessment was $25
x=$400/$25=16 There were originally 16 members.
CHECK 
ax=(x+4)(a-$5)=$400
$25(16)=20($20)=$400
$400=$400=$400