She worked as a carpenter for 12 hours and as a blacksmith for 18 hours.

Assuming you mean she earned $20 as a carpenter and $25 as a blacksmith per hour, with a total of 30 hours for $690,
let c represent carpenter hours and b for blacksmith hours.

20c + 25b = 690
c + b = 30
Subtract b from each side so that c = 30 - b
Plug this value into the first equation

20(30 - b) + 25b = 690
600 - 20b + 25b = 690
600 + 5b = 690
5b = 90
b = 18

To find c, plug this value of b into the other equation
c + 18 = 30
c = 12

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3 years ago

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In how many ways can a quality-control engineer select a sample of3 transistors for testing frm a batch of 100 transistors?

Advocard [28]

Answer:

161700 ways.

Step-by-step explanation:

The order in which the transistors are chosen is not important. This means that we use the combinations formula to solve this question.

Combinations formula:

$C_{n,x}$ is the number of different combinations of x objects from a set of n elements, given by the following formula.