try this:
Interest payout by percent Int (var) = 0;
Interest payout by percent Int (var) = Math.floor((Math.random() * 2) + 1);
a = $gameVariables;
accumulated interest (var) += math.round(a.value(bank money(var))/100*a.value(Interest payout by percent Int (var));
bank money (var) += accumulated interest (var);