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Three circuits' insulation resistance in parallel200, 50 and 40 megohms together. What reads?
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What would you read if all three circuits were tested together?
Answer: 20 MΩ.
Testing circuits together puts their insulation in parallel. Golden rule: the total of resistances in parallel is always lower than the lowest one. The lowest here is 40 MΩ, so anything above that is out straight away, including 290 MΩ, which is just the three added together. If only one option is below the lowest, that's your answer. Otherwise, work it out.
Method 1: the x⁻¹ button (quickest)
200 x⁻¹ + 50 x⁻¹ + 40 x⁻¹ = → 1/20 (S⇔D → 0.05) Ans x⁻¹ = → 20 Don't stop at 0.05! That's 1/R. Press x⁻¹ once more. Answer: 20 MΩ
Method 2: the long way
1 ÷ 200 = 0.005 1 ÷ 50 = 0.02 1 ÷ 40 = 0.025 0.005 + 0.02 + 0.025 = 0.05 1 ÷ 0.05 = 20 MΩ
On older Casios it's a labelled key next to sin. On newer Casios it's awkward: you press SHIFT and the key directly under the D-pad, and it isn't clearly labelled. Use the S⇔D key to change a fraction to a decimal.
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