Lisa gives her little brother Sam a 15-second (sec) head start in a 300-meter (m) race. During the race. Sam runs at an average speed of 5 m/sec and Lisa runs at an average speed of 8 m/sec, not including the head start. Since the time Lisa started running, what is the best approximates the number of seconds that had passed when Lisa caught up to Sam?

Created: 1 year ago | Updated: 1 year ago

Updated: 1 year ago

$S a m r u n s a h e a d o f L i s a i n 15 s e c o n d s = 5 x 15 = 75 m H e r e, L i s a' s a v e r a g e s p e e d = 8 m s^{- 1} A n d S a m' s a v e r a g e s p e e d = 5 m s^{- 1} ∴ L i s a' s s p e e d i s = 8 - 5 = 3 m s^{- 1} m o r e t h a n t h a t o f S a m$