Rats dig 3.5 cm on the first day, and then dig 3.5×2=7 cm every day.

Rats dig 40 cm on the first day, and then dig 40÷2=20 cm every day.

This is equivalent to the problem of building roads, starting from both ends and when to build roads. This is digging a hole, starting from both ends, and when to dig the hole through.

Time = total working speed.

The speed is the length that the big mouse digs every day+the length that the small mouse digs every day (starting from the next day, because their speed on the first day is different from that after).

The total workload is 3 10 cm -3.5-40. Subtract the length of the first day from the total length to get the remaining length. How many days will they dig at that rate?

Finally, the most crucial step was found, plus 1. Because the beginning of the first day is not counted, the first day of the trip should be added.

I also teach fourth-grade mathematics, so let's discuss it together.

Is there a mistake in the number? A mouse can dig 3.5 centimeters a day, and a mouse can dig 40 centimeters? Of course, this is beside the point.