Assumption: A person carrying the torch illuminates the whole bridge.

Representation:

b2 – destination end
b1 – starting end

people – p1 p2 p7 p10 (number denotes time taken for crossing)

Solution: 13 mins.

At t = 0 min,

p1, p2 start journey to cross towards b2 from b1 (p1 has the torch)

At t = 1 min,

p1 reaches b2 and starts back towards b1. (has the torch)
p2 in bridge (1 min)

At t = 2 mins,

p2 reaches b2.
p1 reaches back to b1.
p7, p10 start towards b2. (p7 has the torch)

At t = 9 mins,

p2 in b2
p7 reaches b2
p10 in bridge (7 mins into the journey)

p2 starts towards b1 (has the torch)

At t = 11 mins,

p7 in b2
p10 in bridge(9 mins)
p2 reaches b1
p1 in b1

p2 starts towards b2 (has the torch)

At t = 12 mins,

p7 in b2
p10 reaches b2
p2 in bridge (1 min) (has the torch)
p1 starts towards b2

At t = 13 mins,

all reach b2.