Question

find the sum of 15+20+25+....+75​

Answer 1

Answer:

135 is the right answer.......

Answer 2

Explanation:

Notice that if we match up the first and last pairs in the sequence, we always have a pair that adds to

55

:

,

15

,

20

,

25

,

30

,

35

,

40

,

45

,

50

We see that:

5Explanation:

Notice that if we match up the first and last pairs in the sequence, we always have a pair that adds to

55

:

5

,

10

,

15

,

20

,

25

,

30

,

35

,

40

,

45

,

50

We see that:

5

+

50

=

55

−−

10

+

45

=

55

−−

15

+

40

=

55

−−

20

+

35

=

55

−−

25

+

30

=

55

−−

Thus, the sum of the entire sequence is just

5

groupings of

55

, which is

5

×

55

=

275

+

50

=

55

−−

10

+

45

=

55

−−

15

+

40

=

55

−−

20

+

35

=

55

−−

25

+

30

=

55

−−

Thus, the sum of the entire sequence is just

5

groupings of

55

, which is

5

×

55

=

275

.