If Richard mows 1/3 of his yard in 1/2 hours, then Richard can mow 2/3 of his yard in 1 hour.
Answer:

Step-by-step explanation:
A straight angle is equal to 180° degrees. Since both 9x +4 and 3x -18 is supplementary then I can make this claim below

All we have to do now is to solve for x

The empirical rule tells you 68% of normally-distributed observations are within 1 standard deviation of the mean.
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This is a number you may want to memorize, along with the numbers for 2 (95%) and 3 (99.7%) standard deviations.