Express the ratios in fractions 6:8=6∕8 r:48=r∕48 ⇒6∕8=r∕48 Cross multiply them. The numerator at the left multiplies the denominator at the right and the denominator at the left also multiplies the numerator at the right 6×48=8×r Divide both sides by 8 to make r the subject (stand alone) 6×48∕8=r 6×6=r NOTE: 8 goes into 48, 6 times 36=r ⇒r=36