S = 32 and t = 45
 
You replace the letters into numbers…

Since (st) is the same thing as (s × t), it would be 32 × 45, which equals to 1,440.

Then replace the r with 5, so now it’s (6 × 5), instead of (6r).

st ÷ (6r) → 1,440 ÷ 30 = 48

Value of st ÷ (6r) = 48