Answer: 
Step-by-step explanation:
You must draw a right triangle as the one attached, where "x" is the lenght in feet of the ramp.
You need to use the following Trigonometric Identity:

In this case you can identify that:

Now you must substitute these values into
:

Finally, you must solve for "x" in order to find its value.
This is:

4 times 7 = 28
3 times 7 = 21
It will take him 21 days to make 28 bird houses.
X=7
100-30=70
70/10
x=7
hope this helps
Absolute value is the magnitude of a number without regard to its sign. for example, the absolute value of 5 is 5, and the absolute value of -5 is also 5!