Rohan keeps a total of 90 guppies in 4 fish tanks.

There is 1 more guppy in the 2nd tank than in the 1st tank.
There are 2 more guppies in the 3rd tank than in the 2nd tank.
There are 3 more guppies in the 4th tank than in the 3rd tank.
How many guppies are in the 4th tank?
$\textbf{(A)}\ 20 \qquad \textbf{(B)}\ 21 \qquad \textbf{(C)}\ 23 \qquad \textbf{(D)}\ 24 \qquad \textbf{(E)}\ 26$
Let $x$ denote the number of guppies in the first tank.
Then, we have the following for the number of guppies in the rest of the tanks:

$x + 1 =$ the number of guppies in the second tank
$x + 1 + 2 =$ the number of guppies in the third tank
$x + 1 + 2 + 3 =$ the number of guppies in the fourth tank
The number of guppies in all of the tanks combined is 90, so we can write the equation
$x + x + 1 + x + 1 + 2 + x + 1 + 2 + 3 = 90$.
Simplifying the equation gives
$4x + 10 = 90$.
Solving the resulting equation gives $x = 20$, so the number of guppies in the fourth tank is $20 + 1 + 2 + 3 = 26$.
Therefore, the correct answer is $\boxed{\textbf{(E)}\ 26}$.
- C. Ren
~andliu766 (Minor edits)