A no-calculator live case is not unusual. The math is generally designed to be solvable with structured arithmetic, estimation, percentages, ratios, and clean division rather than long technical calculations.
Your priority should be accuracy plus communication. Write the numbers clearly, state the operation before calculating, and use sensible rounding when the case allows it.
Do not overreact by practicing obscure mental-math tricks for hours. Spend more time becoming comfortable with the types of arithmetic that appear naturally inside cases and with checking whether the answer makes business sense.
If the recruiter gives a specific rule for scratch paper or calculators, follow that instruction rather than assuming another office's process will be identical.