This is the concept of scale factors. Given that Joaquin has reduce the dimensions of a picture by scale factor of 0.3 and the original dimension was 10 cm length by 10 cm width, the new dimension will be:
New length=(old length)×(scale factor)
New width=(old width)×(scale factor)
Plugging the values in the above formulas we get:
New length=15x0.3=4.5 cm
New width=10×0.3=3 cm