Step-by-step explanation:

According to the first condition,

Let the initially monthly salary be 100x

Initially spend = (100x) × 80% = 80x

Initially saving = 100x - 80x = 20x

According to the second condition,

Increased monthly salary = 120% × 100x = 120x

Now, spend = 120% × 80x = 96x

Saving = 120x - 96x = 24x

The percentage increase in his savings

⇒

⇒

⇒

⇒ 20%

Answer:

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Let's assume Jack's salary in November was "x" dollars.

In November, Jack spent 80% of his monthly salary, which means he saved 20% of his salary. Therefore, the amount he saved in November is 0.2x dollars.

In December, Jack's monthly salary increased by $900, so his new salary is "x + 900" dollars.

In December, Jack saved 15% of his new monthly salary, which means he saved 0.15(x + 900) dollars.

Since Jack saved the same amount of money in both November and December, we can set up the following equation:

0.2x = 0.15(x + 900)

Now, let's solve this equation to find the value of x, which represents Jack's salary in November:

0.2x = 0.15x + 0.15(900)

0.2x = 0.15x + 135

0.2x - 0.15x = 135

0.05x = 135

x = 135 / 0.05

x = 2700

Therefore, Jack's salary in November was $2700.

To find his salary in December, we can substitute this value back into the equation:

x + 900 = 2700 + 900

x + 900 = 3600

Hence, Jack's salary in December was $3600.