Why can the sum of even numbers in 1-100 be equal to the sum of numbers in 1-50 multiplied by 2

Why can the sum of even numbers in 1-100 be equal to the sum of numbers in 1-50 multiplied by 2

Even numbers in 1-100 are added to 2 + 4 +. + 98 + 100, and the common factor 2 is extracted
=2(1+2+3+.+50)