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If an object 1m tall casts a shadow 3m long then a right-angled triangle can be used to represent this.
Another triangle can also be drawn but with the opposite side being 2.5m. Using the rule of congruent triangles you can therefore deduce that the new shadow is 7.5m long.
Alternatively you could look at the problem from the basis that the object's new height is 2.5 times greater and therefore the shadow must also be 2.5 times longer.