I just guessed and it is 108
The half life of a decaying substance is defined as the time at which the substance decreases to one-half of its original value. In this case, given hlf-life of 10 years and an original mass of 50 grams, after 10 years, the isotope mass becomes 50*0.5 or 25 grams. Answer is B.
The filling rate of the first tank is this:

The filling rate of the second tank is this:

Add them to get the combined rate:

Make a common denominator by multiplying the first fraction by 2/2:

Adding them together, we get:

So, now we know that the combined rate is 3 tanks per 2 hours... but we need to know how long it will take to fill only one tank. So, we need to find the unit value, which we do by doing this:

Then we can cross-multiply:

And divide both sides by 3 to get our final answer:

So, it takes 2/3 of an hour, or 40 minutes.
Hope I helped, let me know if you have any questions :)
A. (3x7)+(3x4)
should be the correct one bc the other options dont get the same answer in the end