(3+ 1) means that mothers rest every 4 days 1 day.
Multiplication is actually the least common multiple of 5 and 4, that is, every 20 days, parents can take Xiaoming out to play the rest. But we have to consider Xiao Ming's situation.
Xiao Ming's rest is a normal calendar, so consider when the time node meets the weekend on the 20th.
March 6th is Sunday, 20 days later, that is, March 26th is Saturday, 20 days later, April 15, Friday. From here, we can already see the rules, which will reduce the number of days every 20 days and a week. So on Sunday after six weeks, parents can take Xiaoming out to play again, and then the next 20 days will be Saturday. That's enough.
To sum up, the logic is that the coincidence period of parents' rest days is 20 days, which is just less than three natural weeks 1 day. In other words, every three natural weeks, parents can synchronize once, but the days in a week will be reduced by one day. Therefore, March 6th is Sunday, and the next cycle happens to be Saturday. That is to say, on average, every 120 days, parents can take Xiaoming to play 1 time, and they can play again after 20 days, and then wait 120 days.