public function convertBaseRateToCurrency(string $currency) : Rates
    {
        if (!empty($this->rates)) {
            if ($this->base !== $currency) {
                if (!empty($this->rates[$currency])) {
                    $usdrate = $this->rates[$currency];
                    foreach ($this->rates as $key => $rate) {
                        $newrates[$key] = $rate * (1 / $usdrate);
                    }
                }
            }
        }
        if (!empty($newrates)) {
            $this->rates = $newrates;
            $this->base = $currency;
        }

        return $this;
    }