Explanation:
the simplest way for me is
1. to convert both terms into full fractions (not mixed numbers).
2. then to bring both fractions to the same denominator.
3. and then do the actual calculation.
and if needed, we can
4. then convert the result back into mixed numbers.
1.
15 1/2 = (15×2 + 1)/2 = 31/2
1 11/20 = (1×20 + 11)/20 = 31/20
2.
we need to convert 31/2 into a fraction .../20, so that we can actually combine it with the 31/20.
how do we get from 2 to 20 ? by multiplying by 10.
in order to keep the value of the fraction unchanged, we need to do the same multiplication also to the numerator :
31/2 × 10/10 = 310/20
3.
310/20 + 31/20 = 341/20
4.
341/20 = (17×20 + 1)/20 = 17 1/20
to get this you do an integer division and note the remainder.
341/20 = 17 remainder 1