Fourth proportional and double reduction to unity
Solve problems by the method of ratios and by reduction to unity, and problems with two changes. Section: Grade 4, fourth proportional and double reduction to unity; motion and joint work are in another studio.
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Guide
In Grade 4 pupils solve problems where three numbers are known and the fourth is wanted (fourth proportional). Two methods work: the method of ratios and reduction to unity.
Method of ratios: 3 tablecloths need 10 m of fabric; how many metres for 6? There are 6 : 3 = 2 times more cloths, so 2 times more fabric: 10 · 2 = 20 (m), or 10 · (6 : 3) = 20 (m).
Reduction to unity: first find the value for one unit, then for the wanted count. In double reduction two quantities change; change one at a time, never both together.