The three calls have equally spaced strikes (ΔK=10) and prices C1=4, C2=3.5, C3=2.75. A no-arbitrage condition for call options is that the price as a function of strike must be convex: C1+C3≥2C2. Here 4+2.75=6.75<7=2×3.5, so convexity is violated, creating an arbitrage.
Construct a butterfly spread: buy one call at strike 1000, buy one call at strike 1020, and sell two calls at strike 1010. The net premium is
C1+C3−2C2=4+2.75−2(3.5)=6.75−7=−0.25,
so the position generates an upfront credit of 0.25.
The payoff at expiration ST is
V(ST)=max(ST−1000,0)+max(ST−1020,0)−2max(ST−1010,0).
Evaluating piecewise:
- ST≤1000: all options expire worthless, V=0.
- 1000<ST≤1010: V=(ST−1000)≥0.
- 1010<ST≤1020: V=(ST−1000)−2(ST−1010)=1020−ST≥0.
- ST>1020: V=(ST−1000)+(ST−1020)−2(ST−1010)=0.
The payoff is non-negative for all ST and strictly positive for 1000<ST<1020. Since the position was entered at a net credit of 0.25, the guaranteed profit is 0.25 dollars (25 cents).