Find the number in the problem with the smallest number of significant figures. Then use pi with at least one more sigificant figure.

To find the radius using the equation V = pi r^2 h you must have everything in the same units. So yes, convert volume from L to mL. Then 1 mL = 1 cm^3. Then convert the height in inches to cm. Then solve the equation for r and the answer will come out in cm. Check the units of the calculation to make sure the units have been treated properly.

It's confusing! To understand if a number has infinite siginicant figures you have to understand what the number represents. If it just a number like 20 (without a decimal point), and we don't know what measurement it represents then it has 1 significant figure. If we are measuring something like birds flying overhead and we are really not sure if it is 21,22,23,24 etc. birds then we might say about 20 (with no decimal point). The last place has no significance because we are really not certain. We only know it is about 20 birds. But if a problem gives you twenty numbers and asks you to find an average, well then we know there are exactly 20 numbers. There is no chance the number is 21 or 20.001 because we can count the twenty numbers, and we are certain there are exactly twenty. In that case the number twenty has an infinite number of significant figures.

I see that you have answerd the first part of the problem correctly. Now all you have to do is figure out the sig figs. Remember to use the same number of sig figs as the number in your calculation with the least number of sig figs. In looking at all the numbers used like height, volume, pi, it would seem to me that the height of 10 inches only has 1 sig fig. That is what limits the accuracy of the calculation. Shouldn't you round to 1 sig fig? I think so! Don't be confused into thinking that you convert 10 inches to cm and then somehow magically it now has more sig figures. 10 inches begins its life with 1 sig fig and can only have 1 sig fig after it is converted to cm.

Say you have a number like "15.00". Would that number have two or four significant figures?

Don't think of numbers in terms of what they could be rounded to. Imagine the following story. Our job is to measure the length of some piece of machinery and report that number in the company magazine. We get a cheap yardstick graduated in 0.5 in and and we make the measurement and find about 15 inches. We report 15. inches. Now the boss says OK fine but we need a more accurate measurement. Go by an expensive set of calipers and make the measurement to 1/1000th of an inch. We do that and after measuring again we find exactly 15.000 The boss is pleased! But what number do we report in the magazine? the number 15.000 could be rounded to 15, but they are not the same thing! You see an employee who reads the magazine and sees the number 15.000 knows that the device has a length between about 14.999 and 15.001 inches. The zeros certainly are significant.

You can only access that information if you have an Adobe Acrobat reader. You can download that software free of charge at www.adobe.com