We can see that resistors R2 and R3 are in series, so their equivalent resistance is the sum of their resistances:
$R_{23} = R_2 + R_3 = 6.38\,\Omega + 15.95\,\Omega = 22.33\,\Omega$
Now, resistors R1 and $R_{23}$ are in parallel, so their equivalent resistance
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