As "milk" appears in both ratios you need to create equivalent ratios with the same value of "milk" in each; then you can combine the two ratios in to one ratio for all three and use that ratio to solve the problem.Equivalent ratios are like equivalent factions: whatever you multiply/divide one side of the ratio by, you must multiply/divide the other side by the same amount and you will have an equivalent ratio - this is how ratios are simplified.The first ratio plain : milk = 2 : 3The second ratio milk : white = 7 : 2To avoid fractions, use the lcm of the value of the "milk" in both ratios: ie the lcm(3, 7) = 21 as the value of "milk" in the equivalent ratios - divide the lcm by the current value to find what number you need to multiply the ratio by:The first ratio can be multiplied by 21 ÷ 3 = 7 to give:plain : milk = 2 : 3 = 2×7 : 3×7 = 14 : 21The second ratio can be multiplied by 21 ÷ 7 = 3 to give:milk : white = 7 : 2 = 7×3 : 2×3 = 21 : 6The two ratios can now be combined in a single ratio with three terms as the "milk" is 21 in each case, forming:plain : milk : white = 14 : 21 : 6From this we can find how many of each type of chocolate there are, and in particular how many plain ones there are:There are 14 + 21 + 6 = 41 parts→ each part is worth 123 chocs ÷ 41 parts = 3 chocs per part→ There are 14 parts × 3 chocs per part plain = 42 plain chocolates-------------------------------------------------------------------To check we can also work out the quantity of the other two types of chocolates:& There are 21 parts × 3 chocs per part milk = 63 milk& There are 6 parts × 3 chocs per part white = 18 whiteThis gives a total box of 42 plain + 63 milk + 18 white = 123 chocolatesAnd recalculate the ratios, simplifying them:Ratio of plain : milk = 42 : 63 = 2×21 : 3×32 = 2 : 3Ratio of milk : white = 63 : 18 = 7×9 : 2×9 = 7 : 2