Ibrahim spends a quarter of his monthly salary on school fees, one-ninth on water and electricity bills, two thirds of the remainder on house up keep and saves the rest. The difference between what he

Fraction spent on school fees = $\frac{1}{4}$.
Fraction spent on water and electricity bills = $\frac{1}{9}$.

Amount spent on house upkeep = $\frac{2}{3}$ of the remainder.

Amount saved = $\frac{1}{3}$ of the remainder.

The difference is Kshs. 10,350.

To subtract, find a common denominator (108).

Ibrahim's monthly salary is $\boxed{\text{Kshs. 48,600}}$.

2. Lessons in Lake Primary and Kisumu Girls High Schools take 30 minutes and 40 minutes respectively. The two bells ring simultaneously at 7.50 a.m. How many times will they ring together again between 7.50 a.m. and 3 p.m.?

Bell 1 rings every 30 minutes.
Bell 2 rings every 40 minutes.
Prime factorization:
$30 = 2 \times 3 \times 5$
$40 = 2^3 \times 5$
LCM$(30, 40) = 2^3 \times 3 \times 5 = 8 \times 3 \times 5 = 120$ minutes.
The bells will ring together every 120 minutes, which is 2 hours.

Start time: 7:50 a.m.
End time: 3:00 p.m.
Duration from 7:50 a.m. to 3:00 p.m.:
From 7:50 a.m. to 8:00 a.m. = 10 minutes.
From 8:00 a.m. to 3:00 p.m. = 7 hours.
Total duration = 7 hours and 10 minutes.
Convert to minutes: $7 \times 60 + 10 = 420 + 10 = 430