Let me preface the answer by saying I'm not a professional mathematician or
skilled puzzlist. In anyone wishes to correct my explanation please feel
free to do so.
There are 2 calculations involved.
First how much bigger is the big bar compared to the regular bar?
(42 g - 24 g) is what per cent of 24 g?
(42 g - 24 g) = x % * 24 g
18 g = x % * 24 g
18 g / 24 g = x %
0.75 = x %
0.75 * 100 = x * 100
75 % = x
The big bar is 75 % bigger than the regular bar. Note that this is
individually -- 1 big bar is 75 % bigger than 1 regular bar.
Yet the box says the big bar is 60 % bigger than the regular bar by weight.
The 60 % figure must refer to the difference between a bar from a entire
box of each size of bar.
There are 8 bars in the regular size box: 8 * 24 g = 192 g.
There are 5 bars in the big size box: 5 * 42 g = 210 g.
The big size box has 210 g - 192 g = 18 g more product. These 18 g
are distributed over 5 bars (18 g / 5 bars = 3.6 g per bar).
Secondly how much bigger is the big bar compared to the regular bar taking
into account an entire box of 5 big bars versus an entire box of 8 regular
bars? Subtract out 3.6 g from the big bar.
((42 g - 3.6 g) - 24 g) is what per cent of 24 g?
(38.4 g - 24 g) = x % * 24 g
14.4 g = x % * 24 g
14.4 g / 24 g = x %
0.6 = x %
0.6 * 100 = x * 100
60 % = x
The 60 % value is a comparison between an entire box of bars and not
between individual bars.