I've been given the following task: "Let's suppose you have an hourglass that measures 7 minutes and an hourglass that measures 4 minutes, and you need to time 9 minutes by using only these hourglasses."
I've already figured out a solution, but that's the easiest part, for the task is not only to find a solution but also to explain it in Russian, and I'm at a loss as to how to express the very first step, which is to start both hourglasses at the same time.
Запустить оба песочных часов? Запустить обе песочных часов?
Sounds weird and I guess neither of those variants is grammatically correct, for a single hourglass is a plural noun in Russian, whilst the Russian word for "both" requires a singular noun (оба стакана, not оба стаканов).
But how do I say? Is it only silly me who can't figure out the right endings?
I tried to circumvent the problem by using "two" instead of "both": запустить два песочных часов, запустить две песочных часов. But it seems I'm getting exactly the same problem.
I also thought about using "couple" instead of "both": запустить пару песочных часов. I'm unsure whether this variant is grammatically correct, but even if it is, I think it sounds rather like "start a couple of hourglasses," whilst we have the two hourglasses, the ones given. I could try to avoid that by saying запустить нашу пару песочных часов or запустить эту пару песочных часов, but I can't get rid of the feeling of weirdness of using the word "couple" and such constructions in this context.
Desperate, I thought also about запустить все песочные часы, but there are not many hourglasses, just two, so it's weird again. I guess no native Russian speaker will say запустить все песочные часы in this situation.
My mind is overwhelmed and Russian is hard!
Can you help me put my chaotic thoughts in order? Is there any natural way to say "start both hourglasses" in Russian, after all?