You made 3/20 of the 400 cards, which is 60 cards.
The simultaneous equations is missing +8
The answer would be the sequence has a common ratio of 4.
The explanation for this would be:
To get the common ratio for get the simplest form of the two numbers, so 12/3 = 4 and 48/12 = 4So we know that their common ratio is 4.
To check:
3 x 4 = 12
12 x 4 = 48
48 x 4 = 192
192 x 4 = 768
The work and answer provided are correct, the simpler way of doing it is
1)
1/6 page written in 1/12 of a minute
to write a full page, you would need to write 1/6, 6 times
so,

and

so it takes 1/2 of a minute to write a full page, there are 60 seconds in a minute, 1/2 of 60 seconds is 30 seconds
same process works for question 2(use 3 instead of 6 though)