[racket] cond and else together

Using DrRacket v5.3.6, if I enter:

(define (tax pay)
  (cond
    ((<= pay 240.0) 0)
    ((<= pay 480.0) (* (/ 15 100) pay))
    ((else ((+ (* (/ 15 100) 480) (* (/ 28 100) (- pay 480))))))))

I get error

else: not allowed as an expression in: (else ((+ (* (/ 15 100) 480) (* (/
28 100) (- pay 480)))))

Can you not have an expression in an else clause?  Can an else only contain
a simple value?  What am I doing wrong?

This is HTDP ex 4.4.2 by the way.
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