Expressions, unknowns and rules

Work out $5 + 4 \times 3$.

Work out $(8 + 4) \div 2 \times 3$.

Find the unknown: $\square + 17 = 40$.

Find the unknown: $45 = \square \times 5$.

Tendai thought of a number. He doubled it, then added 6, and got 20. What was his number?

Which rule fits every row of this table?

Where should the brackets go to make this true?

$2 + 3 \times 4 - 1 = 19$

At a stall, one pencil costs $p$ shillings and one rubber costs $r$ shillings. What does $3 \times p + 2 \times r$ mean?

In Lean, ↔ means "exactly when", ∀ x : Nat means "for every whole number $x$" and ∃ x means "there is a number $x$". Which statement says that 23 is the only answer to $\square + 17 = 40$?