7 slow days
You know the worker worked a total of 20 days, some at $80 per day, and some at $40 per day. After the 20 days, the worker was paid $1320.
x = days paid full time ($80)
y = days paid half time ($40)
you have two unknowns, so you'll need two equations to solve, from the problem statement, we can derive the following relationships:
($80)x + ($40)y = $1320
x + y = 20 days
x = 20 - y
80(20 - y) + 40y = 1320
1600 - 80y + 40y = 1320
1600 - 40y = 1320
1600 - 1320 = 40y
280 = 40y
y = 280/40 = 7
thus 7 days paid half time (i.e. slow days)
verify your solution is correct
y = 7
x = 20 - 7 = 13
80(13) + 40(7) = 1320 [OK]
