21.1. A motorcyclist set out from city A to city B. If he travels at a speed of 50 km per hour, he will arrive in B one hour earlier than the scheduled time. If he travels at a speed of 35 km per hour, he will be two hours late. Find the distance between cities A and B.

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(7-8 \text { th grade })
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21.1. Let $A B=x$. Then we easily arrive at the equation: $x: 50+1=x: 35-2$, from which $x=350$. A n s w e r. $A B=350$ km.