Workload completed by two people in 7 hours: (1/50)×2×7= 14/50,

Workload completed by the first person: 1- 14/50=36/50.

Working hours of the first person: 5+7= 12.

Number of people who do it first: (36/50) ÷ [(1/50) ×12] = 3 (people).

A: Three people will be arranged to do it first.