Filling in 16 for a, we have ...
... lower limit ≤ heart rate ≤ upper limit . . . . . . . this is the meaning of upper and lower limits
... 0.5×(220 -16) ≤ heart rate ≤ 0.9×(220 -16) . . . . . substitute given expressions
... 0.5×204 ≤ heart rate ≤ 0.9×204 . . . . . . . . . . . . . begin expression evaluation
... 102 ≤ heart rate ≤ 183.6