Mark's daily pocket money is 40 cents more than jee sian's. Each of them spends $1.40 a day and saves the rest. By the time Jee Sian has saved $50, Maek has saved $20 more than Jee sian. How much is Jee Sian's daily pocket money?

Assign variables to the pocket money of Mark and Jee Sian.

For example,
let m be the daily pocket money of mark,
let j be the daily pocket money of Jee Sian.

From the first line,
m = j + 40

You have to give Jee Sian 40 more cents to get the same amount as Mark.

Each day, Mark spends $1.40, this means, he saves m - 1.4
Each day, Jee Sian spends $1.40, this means he saves j - 1.4

After some days, d, Mark has got $50. The number of days multiplied by the daily savings give the total saved.
d * (m - 1,4) = 50

same for Jee Sian;
d * (j - 1.4) = 20

So that you get two equations which can be combined:

d = \frac{50}{m-1.4}

d = \frac{20}{j-1.4}

\frac{50}{m-1.4} = \frac{20}{j-1.4}

And with two equations and two variables, can you solve for m and j, the daily pocket money of Mark and Jee Sian? :)