Certain steps are still confusing...
eg... why no. of units left = 4 - 27/7?

That is talking about at the end of the 9th day... There are 4 units at the start, and 27/7 units have been spent by the end of the 9th day


Alternatively,
Fraction of $ spent in the first 7th days
= 1 - 1/4
= 3/4

3/4 = 21/28 (multiply both denominator and numerator by 7)

You can see this as 21 units were spent in the first 7 days , and there are 28 units in total.

So in 1 day, $ used
= 21 units ÷ 7
= 3 units


After the 9th day,
Total amount spent = 9 x 3 units = 27 units

Amount left = 28 units - 27 units
= 1 unit

1 unit = $10

Total at first = 28 units
= 28 x $10
= $280

The trick here is to multiply both the denominator and numerator of the fraction by 7 so that when we divide by the 7 days, we get a whole number for the number of units.


The other tutor has the same concept, but in fractions. This method here is by visualising in units. Pick the one that is more easy to understand for exam answering.