plz come who interested in Maths (2)

(a)
let S is the saving
I=C+kN (where k is contant)
S = 5/8(I - 6400)
If he sells 250items, he can save $5375 in that month.
5375 = 5/8(C + 250k - 6400)
C + 250k = 15000 ------------------ (1)
Also, If he sells less than 35 items, he cannot afford the rent

C + 35k = 6400 ----------- (2)

solving, k = 40, C = 5000
substitute N=250

I = 5000 + (40)(250) = 15000
His monthly income in that month is 15000
b)
If he sells 35 items
I=5000 + (40)(35) = 6400
His monthly income is 6400
c)
I = 5000 + 40N
C=5000

d)
Let R be remainings
6400 : R = 5 : 3
5R = 19200
R=3840

I-C = 3840
5000+40N-5000=3840
40N=3840
N=96
So he must sell 96 items so that the rent and the remainings is also in the ratio of 5:3.