Answer:
12 hours
Step-by-step explanation:
Let Theo's siste can paint a fence in x times
Given.
Theo can paint a fence three times as fast as his sister can. Working together, they can paint a fence in 9 hours.

Apply cross multiplication

Theo’s sister to paint the fence in 12 hours.
Brianna's thinking is wrong because obviously all of the expressions are going to equal -4 when x is 0 because -4 would be the only value. Also, if x was a different number, the expressions wouldn't be equivalent. The equivalent expressions are A. 9x - 3x - 4, and C. 5x + x - 4. This is because when both are simplified, they equal 6x - 4.
7 batches of cookies 14 x 7=98 whereas sandy needs 12 x 8=96 ,so 7 batches would be perfect