It is related to econometrics, however using simple algebraic logic we can solve this problem. General formula with residual is

, where r is a residual. Plugging the pair into the formula, we obtain that

and r=0.5
the expression 30 tens - 3 tens 3 tenths. A number proceeded by "tens" means that the number is multiplied by 10 while "tenths" means that the number is divided by 10. 30*10 - 3*10*3/10 equal to 300 - 9 or 291.
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