In the morning, Xiao Zhang drove from A to B. At 1 o'clock in the afternoon, Xiao Wang also drove from place A to place B. At 2 o'clock in the afternoon, the distance between the two is 15 kilometers, and at 3 o'clock in the afternoon, the distance between the two is still 15 kilometers. Xiao Wang arrives at B at 4:00 p.m., and Xiao Zhang arrives at B at 7:00 p.m. What time does Xiao Zhang leave in the morning? (Hint: you can draw a line diagram to help solve the problem)
早晨，小张开车从甲地出发去乙地．下午1点，小王开车也从甲地出发，前往乙地．下午2点时，两人之间的距离是15千米，下午3点时两人之间的距离还是15千米．下午4点时小王到达乙地，晚上7点小张到达乙地，小张是早晨几点出发？(提示：可以画线段图帮助解题)
<gadget id="calculator">15 + 15</gadget>
<output>30</output>

<gadget id="calculator">4 - 3</gadget>
<output>1</output>

<gadget id="calculator">30 / 1</gadget>
<output>30</output>

<gadget id="calculator">15 + 30</gadget>
<output>45</output>

<gadget id="calculator">7 - 4</gadget>
<output>3</output>

<gadget id="calculator">45 / 3</gadget>
<output>15</output>

<gadget id="calculator">19 - 15</gadget>
<output>4</output>

<gadget id="calculator">4 - 1</gadget>
<output>3</output>

<gadget id="calculator">3 / 15</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">30 * (1/5)</gadget>
<output>6</output>

<gadget id="calculator">4 + 6</gadget>
<output>10</output>

<result>10</result>