ETF or index fund?

An ETF charges 0.04% a year; buying and selling it costs brokerage of 0.03% each way, a half-spread of 0.04% each way, stamp duty of 0.015% on the purchase and an expected 0.05% in premium paid. An index fund on the same index charges 0.18% with entry and exit costs of 0.006%. Gross return 11% (illustrative). (a) Compute the ETF's round-trip cost and the crossover horizon, by the approximation and exactly. (b) A SIP of ₹3,000 a month pays a flat brokerage of ₹20 per purchase instead of the 0.03% on the buy. Recompute the round-trip cost per instalment and the crossover horizon. (c) State a decision rule for the investor in (b).