For this case we have the following variables:
A: Represents the number of adults
S: Represents the number of students
C: Represents the number of children
For the income we have:
-----> (1)
Regarding the number of people at the concert we have:
-----> (2)
If there are 40 more children than students, we have:
-----> (3)
We substitute C in the second equation and it remains:
![A + S + 40 + S = 400](https://img.qammunity.org/2019/formulas/mathematics/high-school/4c4l4wq78m2gscymcyby414z3p6j1c71gy.png)
![A + 40 + 2S = 400](https://img.qammunity.org/2019/formulas/mathematics/high-school/cynz5j76w0rpwl6zx48mpu48sx74lp1gnk.png)
![A + 2S = 400-40](https://img.qammunity.org/2019/formulas/mathematics/high-school/54gybdcctfaythiusylemdz9xzelntn2su.png)
![A + 2S = 360](https://img.qammunity.org/2019/formulas/mathematics/high-school/fh8ziom8fx5p2c0p6wwyxlymfwjz5gujk6.png)
--------> (4)
Substituting (3) and (4) in (1) we have:
![22 (360-2S) + 15S + 13.50 (40 + S) = 7840](https://img.qammunity.org/2019/formulas/mathematics/high-school/cedz4ks851j6znaekmksuyhrpdkwo2s457.png)
![7920-44S + 15S + 540 + 13.50S = 7840](https://img.qammunity.org/2019/formulas/mathematics/high-school/xpzso1xc3366e7f2pteesepye3w7hwvxfx.png)
![7920-7840 + 540 = 44S-15S-13.50S](https://img.qammunity.org/2019/formulas/mathematics/high-school/dqaiatvws5d4g18jipuwwmw7r0r2k4so0r.png)
![620 = 15.50S](https://img.qammunity.org/2019/formulas/mathematics/high-school/g2hh85fjwen8xwn9sbdf2ex4k9uze8zw6v.png)
![S = (620)/(15.50)](https://img.qammunity.org/2019/formulas/mathematics/high-school/yt03fjhvo4a2i7xpdsomxmfstr457sjd8n.png)
![S = 40](https://img.qammunity.org/2019/formulas/mathematics/high-school/6tyebpp32f77t67grl5xmf0i78us2vs916.png)
So, there are 40 children.
Substituting S in (3) and obtaining C:
![C = 40 + S](https://img.qammunity.org/2019/formulas/mathematics/high-school/6a0oa682c6fn0wpwdiv9uqw80o1tjs0gqu.png)
![C = 40 + 40](https://img.qammunity.org/2019/formulas/mathematics/high-school/x0i9d2wn3ooxb0jvopdruk6gx9plmqvpl4.png)
![C = 80](https://img.qammunity.org/2019/formulas/mathematics/high-school/loufprizcaa8kdlgevbsex5vjsdhtb9ian.png)
Substituting S in (4):
![A = 360-2 (40)\\A = 360-80\\A = 280](https://img.qammunity.org/2019/formulas/mathematics/high-school/hhwcwuq3n0fe2kn8u6d411smxtm8kfn1fz.png)
Thus, 280 tickets of adults, 80 of children and 40 of students were sold.
Answer:
280 tickets for adults, 80 for children and 40 for students