Noting that Kimani will travel (15km/hr)*(1.5hrs) = 22.5km before Mwaura starts, we can write the following, setting t = 0 to be noon:
1. Dk_k(t) = 15t + 22.5
2. Dk_m(t) = 40 - 25t
Where each Dk(t) is distance from Kimani's house.
When the two meet, their distances from Kimani's will be equal, so set eq 1 equal to eq 2. Then solve for t to get t = 0.4375hr
At t=0.4375hr, Dk_k(t) = 15(0.4375) + 22.5 = 6.5625 + 22.5 = 29.0625km
Check using Dk_m(t):
at t=0.4375hr, Dk_m(t) = 40 - 25(0.4375) = 29.0625km