You can buy up to 6 tetras.
Work Step by Step
Let t be the number of tetras you can buy. The cost of each tetra is \$3.99, or $3.99t$. If you can spend no more than \$25, the cost of the tetras must be less than or equal to \$25. $3.99t\leq25\longrightarrow$ divide each side by 3.99 $3.99t\div3.99\leq25\div3.99$ $t\leq6.3$ Since it is not possible to buy a fraction of a tetra and rounding up would make the cost exceed \$25, $t\leq6$