[tex][(13 \times a + 2 \times 7) \div 4 \times 100 - 900] \div 10 - 9 = 1[/tex]
[tex][(13 \times a + 2 \times 7) \div 4 \times 100 - 900] \div 10 = 10[/tex]
[tex](13 \times a + 2 \times 7) \div 4 \times 100 - 900 = 100[/tex]
[tex](13 \times a + 2 \times 7) \div 4 \times 100 = 1000[/tex]
[tex](13 \times a + 2 \times 7) \div 4 = 10[/tex]
[tex]13 \times a + 14 = 40[/tex]
[tex]13 \times a = 26[/tex]
[tex]a = 2[/tex]