Problem
In one second, 20 fl oz come in and 4 fl oz leak out, so the water level rises by 20 - 4 = 16 fl oz each second.
Only the leftover after the leak actually stays in the bucket.
3.MD.A.2Analyze The UnitsIn each second, the amount of water in the bucket rises by 16 fl oz.
In one second the faucet adds 20 fl oz and the hole drains 4 fl oz, so the water left standing in the bucket is the 20 that came in with the 4 that leaked out taken away, which is 20 - 4 = 16.
The 20 fl oz that enter during that second split with no gaps or overlap into two parts, the 4 fl oz that leak out and the fl oz that stay, so the leaked part and the staying part together rebuild the whole 20.
Since the leaked 4 and the staying amount add back up to 20, we find the staying amount by taking the 4 out of the 20 again, and subtraction is exactly what undoes that adding.
The bucket holds 128 fl oz and fills 16 fl oz each second, so the time is 128 divided by 16, which is 8 seconds.
Filling 16 fl oz each second, it takes 8 equal seconds to reach 128 fl oz.
3.OA.A.3Look For A PatternThis only needs Grade 3 division once you see each second really adds 20 minus 4 = 16 fl oz!