How do I solve this word problem?

Question

It takes bob 5 seconds to flip a pancake and jerry 8 seconds to do the same. How many seconds does it take them to flip 78 cakes?

Answer 1

find the LCM of 5 and 8 which is 5*8=40

after 40 seconds Bob flips 40/5=8 pancakes

after 40 seconds Jerry flips 40/8=5 pancakes

every 40 seconds together they flip 5+8=13 pancakes

78/13=6

6*40=240 seconds or 240/60=4 minutes

seconds     pancakes

40             13

80             26

120           39

160           52

200           65

240           78