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Standard arithmetic: + - * /. Integer division: // (Lua 5.3+). Modulo: %. Exponent: ^.

print(7 / 2)     -- 3.5  (always float division)
print(7 // 2)    -- 3    (floor division)
print(7 % 2)     -- 1
print(2 ^ 10)    -- 1024.0

Math functions live in the math library:

print(math.floor(3.7))    -- 3
print(math.sqrt(16))      -- 4.0
print(math.max(1, 9, 4))  -- 9
print(math.pi)            -- 3.1415926535898

Convert string to number with tonumber(s). Returns nil if not parseable.