Find the No of moles of calcium hydroxide that must be dissolved to make 250ml solution in pH10. 65?

To do well in chemistry you need to be perfectly prepared in arithmetic.

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.

A pH of 10 means one H+ ion in 10^10 molecules of water, and that means 4 OH- ions. The number of H+ and OH- add up to 14.

A liter of water contains close to 52 moles of water, so 250 ml contains 1/4 of that, or 13 moles. So you want 4 x 13 x 10^-10 moles of CaOH assuming it dissolves entirely.

"Strong" acid or base means it ionizes completely in water. It has nothing to do with concentration or corrosiveness. There are seven strong acids:

Sulfuric
Nitric
Hydrochloric
Chloric
Perchloric
Hydrobromic
Hydroiodic

It's easy to remember the list because there are three of the most common industrial acids, three "chloric" acids, three "hydro" acids, and they all make a clumsy "7" on the periodic table.

There are dozens of strong bases and hundreds of "almost strong" bases. CaOH happens to be a strong base.

I am pretty old and can not reliably do arithmetic in my head any more, so you will need to check all my numbers. And while you are at it, learn all this and make it your own.