1/4 + 1/5 = 1/t
1/4 + 1/5 = 1/t, solve for t
The LCD of 4, 5 = 20
[(1*5) + (1*4) ] / 20 = 1/t
[ 5 + 4 ] / 20 = 1/t
9 / 20 =1/t cross multiply
9t = 20 divide both sides by 9
t = 20 / 9