So, our f(t) is the number of liters burned in t days. If t is 1, f(t)=f(1) and so on for every t.
w(r) id the number of liters in r weeks. This is, in one week there are w(1) liters burned.
As in one week there are 7 days, we can replace the r, that is a week, by something that represents 7 days. As 1 day is represented by t, one week can be 7t (in other words r = 7t). So, we have that the liters burned in one week are:
w(r) = w[7f(t)]
So, we represented the liters in one week by it measure of days.
So, we can post that the number of liters burned in 7 days is the same as the number of liters burned 1 day multiplied by 7 times. So:
w (r) = w[7 f(t)] = 7 f(t)
Here we hace the w function represented in terms of t instead of r.