7.1. In the expression

$$
(* *+*)(* *+*)=* * * *
$$

insert digits instead of asterisks so that a correct equality is obtained and no more than 4 different digits are used. (A number cannot start with zero).
Solution. For example, $(90+9)(10+1)=1089$. Or: $(99+1)(10+9)=1900$. Criteria.

(+.) Correct left side.

$(+)$ Correct answer.