Answer:
558
Step-by-step explanation:
The sum of the first n numbers is ½n(n + 1)
which is the left most number in each row number n
so finding the row containing 526
½n(n + 1) > 526
½n² + ½n > 526
n² + n > 1052
n² + n - 1052 > 0
n > (-1 + √(1² - 4(1)(-1052))) / 2(1)
n > 31.938
so 526 is almost to the end of the 32 row.
The end of that row is ½(32(33)) = 528
so 526 resides two spaces to the right of 528
The end of the row below it, row 33, is ½(33)(34) = 561
As each row is one number longer than the previous row, we count back
2 + 1 = 3 numbers from the end to find what is under 526
561 - 3 = 558
