9. una persona compró 4 mesas y 2 sillas por $200. luego, compró 2 mesas y
sillas por $200. ¿cuánto cuesta cada mesa y cada silla?
tables: $25," chairs: 850

We have the problem:

"Una persona compró 4 mesas y 2 sillas por $200. Luego, compró 2 mesas y  sillas por $200. ¿Cuánto cuesta cada mesa y cada silla?"

Where one data is missing, but in the comments of the question you said that the number of chairs in the second bought is 7, so we have:

Una persona compró 4 mesas y 2 sillas por $200. Luego, compró 2 mesas y  7 sillas por $200. ¿Cuánto cuesta cada mesa y cada silla?

which means:

"One person bought 4 tables and 2 chairs for $ 200. Then, he bought 2 tables and  7 chairs for $ 200. How much does each table and each chair cost?"

So we know that:

4*T + 2*C = $200

2*T + 7*C = $200

Where T is the price of each table and C is the price of each chair.

We can isolate C in the first equation, and get:

2*C = $200 - 4*T

C = $100 - 2*T

and replace it in the second equation to get:

2*T + 7*($100 - 2*T) = $200

2*T + $700 - 14*T = $200

12*T = $500

T = $500/12 = $41.66

Then, we can replace it in the equation:

C = $100 - 2*T =  $100 - 2*$41.66 = $16.66

So each chair cost around $16.66 and each table costs around $41.66

en la escuela tenemos momentos muy cheveres y momentos muy aburridos.