Using the linear equation form
with variable y as the # of arrangements and variable x as time in minutes:
y = mx + b
at x = 0 we were told that y = 108
Substitution yields the equation
108 = 0 + b
and from that we learn the value of b in the solution is b = 108
y = mx + 108
After the substitution for b, we can see all that we need is the value of m.
At x = 15 there is one additional arrangement completed,
therefore at x = 15, y = 109.
Substituting the knowns into our working equation y = mx + 108 yields:
109 = m*15 + 108
Solve it for m. First subtract 108 from both sides of the equation and simplify.
109 - 108 = m*15 + 108 - 108
1 = m*15
Divide both sides of the equation by 15 to get the value for m
1/15 = m
Substitution back into the working equation y = mx + 108 yields:
y = (1/15)x + 108