A positive number when decreased by 4 is equal to 21 times the reciprocal of the number. The number is Choose the correct choice:
A) 3
B) 5
C) 7
D) 9
E) 12
Sol.   Let the numbers be x.   Then, x - 4 = 21/x   ⇔ x2 - 4x - 21 = 0   ⇔ (x-7)(x+3) = 0   ⇔ x = 7.   Answer 7
<result>C</result>
A positive number when decreased by 4 is equal to 21 times the reciprocal of the number. The number is