Video solution:

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Written solution:

Let $x$ be the number of guppies in the 1st tank. Hence, there are $x+1$ guppies in the 2nd tank, $x+3$ guppies in the 3rd tank, and $x+6$ guppies in the 4th tank. We then use the fact that there are a total of 90 guppies in the 4 tanks to find $x$: $$x + x + 1 + x + 3 + x + 6 = 90$$ $$4x+10=90$$ $$x=20.$$

Note that we are not yet done since we are asked for the number of guppies in the 4th tank and not the 1st. There are $x+6=20+6=26$ guppies in the 4th tank.

Thus, E is the correct answer.