1$\frac{1}{21}$ : (15,75 - 15$\frac{1}{4}$) + 2$\frac{1}{12}$ : (7$\frac{3}{4}$ - 7,25)
= 22/21 : (63/4 - 61/4) + 25/12 : (31/4 - 29/4)
= 22/21 : 1/2 + 25/12 : 1/2
= 22/21 x 2 + 25/12 x 2
= (22/21 + 25/12) x 2
= 263/84 x 2
= 263/42
Vậy 1$\frac{1}{21}$ : (15,75 - 15$\frac{1}{4}$) + 2$\frac{1}{12}$ : (7$\frac{3}{4}$ - 7,25) = 263/42