1) (x + 1) + (x + 2) + (x + 3) + ... + (x + 50) = 1375
<=> 50x + (1 + 2 + 3 + ... + 50) = 1375
<=> 50x + [(1 + 50) + (2 + 49) + ... + (25 + 26)] = 1375
<=> 50x + 51.25 = 1375
<=> 50x + 1275 = 1375
<=> 50x = 100
<=> x = 2

2) (x + 1) + (x + 3) + (x + 5) + ... + (x + 55) = 868
<=> 28x + (1 + 3 + 5 + ... + 55) = 868
<=> 28x + [(1 + 55) + (3 + 53) + ... + (27 + 29)] = 868
<=> 28x + 56.14 = 868
<=> 28x + 784 = 868
<=> 28x = 84
<=> x = 3

3) (x + 2) + (x + 4) + (x + 6) + ... + (x + 48) = 696
<=> 24x + (2 + 4 + 6 + ... + 48) = 696
<=> 24x + [(2 + 48) + (4 + 46) + ... + (24 + 26)] = 696
<=> 24x + 50.12 = 696
<=> 24x + 600 = 696
<=> 24x = 96
<=> x = 4

4) (x + 1) + (x + 2) + (x + 3) + ... + (x + 66) = 2607
<=> 66x + (1 + 2 + 3 + ... + 66) = 2607
<=> 66x + [(1 + 66) + (2 + 65) + ... + (33 + 34)] = 3607
<=> 66x + 67.33 = 3607
<=> 66x + 2211 = 3607
<=> 66x = 1396
<=> x = 698/33