20.3. The family approached the bridge at night. Dad can cross it in 1 minute, Mom in 2, the son in 5, and grandma in 10. They have one flashlight. The bridge can only hold two people at a time. How can they cross the bridge in 17 minutes? (If two people are crossing the bridge, they move at the slower of their speeds. It is not possible to cross the bridge without a flashlight. Shining the light from a distance or throwing the flashlight across the river is not allowed.)
20.3. The main idea is that the grandmother should cross the bridge together with the grandson. First, the father and mother go, then the father returns with the flashlight, after that the grandmother and grandson go, then the mother returns with the flashlight, finally, the father and mother cross the bridge. In total, this will take $2+1+10+2+2=17$ minutes.