One large box of birdseed can feed half a dozen parrots for 15 days. for how many days can this same large box of birdseed feed 5 parrots.

Box = b
Parrots = p
b/p = number of days it can support the parrots
Half a dozen = 6 (A dozen is 12)
1/6 = 15
To work out the percentage of '6' '5' is, use the equation
(5/6)*100 = 83.333
Meaning that with one less bird, the box will support the birds for 16.666... (100 - 18.333...) longer
(15/100)*116.666... = 17.4999
The box will sustain the parrots for 17.5 days

40 & 50 in the right corner are the complementary angles as when placed in a sum (added together) they make 90°