One optical scanner can read a stack of sheets in 20 minutes. A second scanner can read the same stack in 12 minutes. How much time would it take for both scanners together to process the one stack of sheets?

Step-by-step explanation:

First scanner can read 1/20 of all sheets in 1 minute

Second scanner can read 1/12 of all sheets in 1 minute

Both scanners can read in 1 minute:

Time to read all sheets:

the solution of the equation is 2.23 ⇒ answer b

step-by-step explanation:

* lets revise the meaning of exponential function

- the form of the exponential function is y = ab^x, where a ≠ 0, b > 0 ,  

  b ≠ 1, and x is any real number

- it has a constant base b

- it has a variable exponent x

- to solve an exponential equation, take the log or ln of both sides,  

  and solve for the variable

* lets solve the problem

∵

- the base is 7 and the exponent is x

- insert ㏑ in both sides

∴

- use the rule

∴ x ㏑(7) = ㏑(77)

- to find x divide both sides by ㏑(7)

∴

* the solution of the equation is 2.23 to the nearest 2 decimal places