Marta expects $50,000 in revenue, but that's not cost, so we can nevermind that.
the "fixed cost" namely overhead just to open the doors is $100,000.
she's expecting 500 folks, each will incur costs of $25 a pop, so for all 500 that'd be 500* 25 or 12500 total.
so Marta's total cost is the overhead and 25 per 500 folks, namely 100,000 + 12500 = 112,500.