First, 2/3 means the whole is broken into 3 equal parts, and you take two
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Now 1/4 of 2/3 means you break your two parts into 4 pieces and take 1.
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However as the denominator of a fraction tells how many "equal" parts are in the whole, you must also break the last third of the original fraction in half so you have equal parts.
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This visual shows you have 1 piece out of 6 equal sized pieces or 1/6
1/4 of 2/3 is equal to 1/6
As stated below, this can be found with just multiplication.