The meaning of '5+10' is 'what number do you get when you add five and ten together'.
When you add two numbers, write the bits of the number under hundreds, tens and units columns. Write one number below the other, with the bits in the right columns. Then add the numbers up in each of the hundreds (H), tens (T), units (U) columns and write the result under the relevant column.
So for '5+10' write
`underline("H" quad "T" quad "U") `
= 1 5
When we add up the numbers in the units (U) column here, we have zero plus five. Anything plus zero is the same as it started, so zero plus five (which is the same as five plus zero) is 5. There is only one ten in the tens (T) column so the sum in that column is 1.
The answer to '5+10' is 15, that is one ten plus five units.
5+10=15. This means that if you add 10 things to the 5 things, how many do you have in all? For example, suppose you have 10 oranges on your table. Then you add 5 more. How many do you have in total? 15.
``The numbers align and add up to 15.