This year

This year 2020 is a leap year. How many leap years there had been be since 1904?

Correct result:

n =  30

Solution:

$$\begin{gathered} d=4 \\ a=1904 \\ b=2020 \\ b=a+(n-1)\cdot d \\ n={\displaystyle \frac{b-a}{d}}+1={\displaystyle \frac{2020-1904}{4}}+1=30 \end{gathered}$$

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