[racket] exercise problem

Hello,

I try to make this exercise :

Some credit card companies pay back a small portion of the charges a 
customer makes over a year. One company returns

 1.

    .25% for the first $500 of charges,

 2.

    .50% for the next $1000 (that is, the portion between $500 and $1500),

 3.

    .75% for the next $1000 (that is, the portion between $1500 and $2500),

 4.

    and 1.0% for everything above $2500.

Thus, a customer who charges $400 a year receives $1.00, which is 0.25 · 
1/100 · 400, and one who charges $1,400 a year receives $5.75, which is 
1.25 = 0.25 · 1/100 · 500 for the first $500 and 0.50 · 1/100 · 900 = 
4.50 for the next $900.

Determine by hand the pay-backs for a customer who charged $2000 and one 
who charged $2600.

Define the function |pay-back|, which consumes a charge amount and 
computes the corresponding pay-back amount

So I made this :

(define (tarief4 amount)
   (cond
     [ ( < amount 2500)(- (* .00100 amount)2500)]
     [ else ( - ( * .0100 1000) 2500)]))


(define (tarief3 amount)
   (cond
     [ ( < amount 2500)(- (* .0075 amount)1500)]
     [ else ( - ( * .0075 1000) 1500)]))

(define (tarief2 amount)
   (cond
     [ ( < amount 1500)(- (* .0050 amount)500)]
     [ else ( - ( * .0050 1000) 500)]))


(define (tarief1 amount)
   (cond
     [ (< amount 500) (* 0.025 amount)]
     [ else (* 0.025 500)]
     ))


(define (payback amount)
   (cond
     [ (<= amount 500) ((tarief1 amount))]
     [ (and ( <= amount 1500) (> amount 500)) (+ (tarief1 
amount)(tarief2 amount))]
     [ (and ( <= amount 2500) (> amount 1500)) (+ (tarief3 amount) 
(tarief2 amount))]
     [ else (+ (tarief4 amount)(tarief3 amount))]
     ))


But now I get this error message :
unsaved editor>:27:23: function call: expected a function after the open 
parenthesis, but found a part in: (tarief1 amount)

When I do (tarief 1 amount) instead of ((tarief amount)) I don't get the 
error message but the outcome of (payback 2600) is not right.

Anyone a tip for me ?

Roelof




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