A 50ml sample of water from a lagoon contain 20mg Ca.what is concentration of Ca in ppm?

physics.nist.gov/cuu/Units/prefixes.html

You just have to memorize this stuff, at least the ones you work with all the time. It's ok to bookmark the page so you can look up the others when you run across them.

There is a prefix every three powers of ten, except for 100, 10, 1/10, and 1/100. To go from one to another you use the LARD rule: LEFT ADD RIGHT DEDUCT. When moving the decimal to the LEFT you ADD to the power of ten. When going to the RIGHT you DEDUCT from the power of ten.

A liter of water equals one kilogram, 10^3 grams. One part per million would be one milligram, 10^-3, per liter, 10^3. If you do your homework this starts to seem obvious.

Get a ruler in your hands. Measure things until you start to understand how a ruler works. Measure some stuff and figure out where the center is. Say you measure a book and it's 7/8" thick. You look at your ruler and see that every eighth is divided into two sixteenths, so obviously half of 7/8" is going to be 7/16". If you write that out you have 1/2 x 7/8 = 7/16. And you notice that 1/2 is divided into 2/4 and then into 4/8 and so on, so you can convert anything to anything by multiplying all the numbers on top and then all the numbers on bottom.

Other rulers are divided into 10 and 100 parts. But an inch is still an inch, so anything on one ruler can be translated to the other ruler. A half inch on one ruler is 5/10 or 50/100 on the other. An eighth inch is just 12.5 marks when you have 100 marks per inch. A metric ruler divides an inch into 25.4 parts, so a half inch would be 12.7 of those parts. Pretty simple, isn't it? Practice this a bit and people will think you went to wizard school.

Now we are ready to address your question: "A 50ml sample of water from a lagoon contain 20mg Ca what is concentration of Ca in ppm?" (Ca does not get a period.)

Parts per million means milligrams per liter. So write it that way: 20mg/(50x10^-3)liter (I spell liter because the character 'l' is ambiguous.) Now you can simply punch that into your calculator.

20e-3/50e-3 = 0.4 ppm

Check: 20 times your sample = one liter, so 20 x 20e-3 = 400e-3 = 0.4e-6 = 0.4 parts per million.

Practice this and you can do this stuff in your head as fast as a person can tell it to you. They literally will think you went to wizard school.