Addition, subtraction and place value to 100

Priya sells 8 mangoes to one customer and 5 mangoes to another. How many mangoes did she sell altogether? ($8 + 5$)

Which of these number sentences are true? Choose all that are true.

What number goes in the box? $9 + \square = 14$

Amina has 12 shells and Lucas has 7 shells. How many more shells does Amina have than Lucas?

Priya has 52 mangoes. What does the 5 in 52 mean?

Which number is bigger, 47 or 52?

Work out $38 + 25$.

Priya has 52 mangoes and sells 27. How many are left? ($52 - 27$)

Gideon's game: true or false? "For any two counting numbers, $a - b$ is the same as $b - a$."

Margaret's computer checked $a + b = b + a$ for every $a$ and $b$ from 0 to 10 — all 121 facts — and found no mistakes. Does that prove the turn-around rule for every pair of numbers, even huge ones?