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There were many correct answers sent in for this problem. As Primary Maths Club (International School of Toulouse) pointed out, it helps if you start counting seconds from the first time the two lights flashed together (at zero seconds).
Some people thought about a number line, others looked for a number that both of the numbers of seconds (4 and 5) would divide into (common multiple). Here are two very well explained solutions.
Holly, Harriette, Caroline, Florence and Rebecca from The Mount School, York:1st light 0 - 4 - 8 - 12 - 16 - 20 - 24 - ......
Christina from Marlborough Primary School :To work this out you need to find a multiple of both 5 and 4 which is 20. So the lights flash together every 20 seconds and to find out how many times they flash in one minute you need to do 60/20 = 3 which means that they flash together 3 times a minute.
Place four pebbles on the sand in the form of a square. Keep adding as few pebbles as necessary to double the area. How many extra pebbles are added each time?
Investigate the different shaped bracelets you could make from 18 different spherical beads. How do they compare if you use 24 beads?
How many different shaped boxes can you design for 36 sweets in one layer? Can you arrange the sweets so that no sweets of the same colour are next to each other in any direction?