Grandpa's door is facing a small dirt hill. The way my grandfather does morning exercises every day is to climb to the top of Xiaotu Mountain and then go down the same road. It is known that the speed of the grandfather climbing the mountain is 60 meters per minute, and the speed of descending the mountain is 90 meters per minute. Ask grandpa what is the average speed of morning exercise every day?
爷爷家门口正对着一座小土山．爷爷每天晨练的方式就是爬上小土山的山顶后随即沿原路下山．已知爷爷爬山的速度是每分钟60米，下山的速度是每分钟90米．求爷爷每天晨练的平均速度是多少？
<gadget id="calculator">1 + 1</gadget>
<output>2</output>

<gadget id="calculator">1 / 60</gadget>
<output>1/60 = around 0.016667</output>

<gadget id="calculator">1 / 90</gadget>
<output>1/90 = around 0.011111</output>

<gadget id="calculator">(1/60) + (1/90)</gadget>
<output>1/36 = around 0.027778</output>

<gadget id="calculator">2 / (1/36)</gadget>
<output>72</output>

<result>72</result>