Explanation:
let :
the job as lifeguard = A
the job as cashier = B
making 2 equations as follows :
(1) A+B = 14 ==> A = 14-B
(2) 10A + 8B = 124
substitute A (equation (1)) into equation (2) :
10(14-B) + 8B = 124
140 -10B +8B = 124
140- 2B = 124
140-124 = 2B
16 = 2 B
B = 8
A = 14-8 = 6
so, as a lifeguard = 6 working hours
and as a cashier = 8 working hours