Let \( x_1, x_2, x_3, x_4 \) be in a geometric progression, If \( 2, 7, 9, 5 \) are subtracted respectively from \( x_1, x_2, x_3, x_4 \), then the resulting numbers are in an arithmetic progression. Then the value of \( \dfrac{1}{24} (x_1 x_2 x_3 x_4) \) is: