Haro has 17 model cars, 31 model trucks, and 9 model airplanes on the shelf in his room. his sister thought it would be funny to hide all the red models. now haro only has 39 models on his shelf. which number sentence could be used to find how many red models his sister hid?
a. (17 + 31 + 9) – r = 39
b. 17 + 31 + 9 + r = 39
c. (17 + 31 + 9) + r = 39
d. 39 + 17 + 31 + 9 = r

Which rational number could be graphed between -4 and -5

The rate of decay of a radioactive substance depends upon the amount present initially. the mass y (mg) of the radioactive substance cobalt-60 present in a sample at time t (years) is represented by the exponential equation y=50e −0.1315 t . answer the following questions in complete sentences. 1. how does the exponential equation above compare to the equation for simple interest that is compounded continuously? explain the similarities. 2. what is the initial amount of cobalt-60 in the sample? 2. how much cobalt-60 is left after 8.4 years? show your work. 3. what would be the y-intercept of the graph? what does it represent? 4. after how many years will the amount of cobalt-60 left be 6.25 mg? explain what happens to the cobalt-60 after 50 years? 5. discuss some “real-world” examples and uses of cobalt-60

This is so hard and it’s for a test