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Jack usually mows his lawn in 3 hours. Marilyn can mow the same yard in 4 hours. How much time would it take for them to mow the lawn together?

Step 1 ：This is a problem of combined work. We know that Jack can mow the lawn in 3 hours and Marilyn can mow it in 4 hours. If they work together, they will be able to mow the lawn faster.

Step 2 ：We can calculate the combined rate of work by adding their individual rates of work. The rate of work is defined as the amount of work done per unit of time. In this case, the amount of work is mowing the lawn, which we can consider as 1 unit of work.

Step 3 ：So, Jack's rate of work is \( \frac{1}{3} \) lawns per hour and Marilyn's rate of work is \( \frac{1}{4} \) lawns per hour.

Step 4 ：The combined rate of work is the sum of these two rates, which is \( \frac{1}{3} + \frac{1}{4} = 0.5833333333333333 \) lawns per hour.

Step 5 ：The time it would take for them to mow the lawn together is the reciprocal of the combined rate of work, which is \( \frac{1}{0.5833333333333333} = 1.7142857142857144 \) hours.

Step 6 ：Final Answer: The time it would take for Jack and Marilyn to mow the lawn together is approximately \( \boxed{1.71} \) hours.