The degrees of freedom is given by the formula

(s_{1}^{2}/n_{1}+ s_{2}^{2}/n_{2})^{2}/ { (s_{1}^{2}/n_{1})^{2}/(n_{1}-1)+ (s_{2}^{2}/n_{2})^{2}/(n_{2}-1)}

This is the formula normally employed

If we plug and simplify , this will work out 15.67 and rounded to 16

for 90% , for 16 degrees of freedom , the t value is 1.746

For detailed calculation, please see.

unequalvar_df.xls

I am not sure what your instructor has briefed.

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