let h= history books
science books = s
h=2s there are 2 times as many history as science so to get them equal we double the science
h+s=81 there are 81 total history and science books
2s+s=81 substitute 2s for h in the above equation
3s = 81 combine like terms
s = 27
There are 27 science books
h=2s
h=2(27)
h=54
there are 54 history books
<h3><u>
Answer:</u></h3>
- Your answer would be "30".
<h3><u>
Step-by-step explanation:</u></h3>
- 2x + 20 = 3x - 10 [Corresponding Angles]
- => 10 + 20 = 3x - 2x
- => 30 = x
<h3><u>Conclusion:</u></h3>
Therefore, your answer would be "30".
Hoped this helped.

<span>respon en los comentario y escribe bien la pregunta para responderte
respond in the comments and write the question well to answer
</span>