Answer: E

Small Hint:

Write each tank amount in terms of the first tank

Big Hint:

The fourth tank has $6$ more guppies than the first

Video solution:

Click to load, then click again to play

Written solution:

Let $x$ be the number of guppies in the $1$st tank. Hence, there are $x+1$ guppies in the $2$nd tank, $x+3$ guppies in the $3$rd tank, and $x+6$ guppies in the $4$th 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 $4$th tank and not the $1$st. There are $x+6=20+6=26$ guppies in the $4$th tank.

Thus, E is the correct answer.