Let us use the equality postulate.
Since each class fills 3/4 bus, we will say:
1 class = 3/4 buses
For us to get the number of classes riding on the 5 1/4 buses for the field trip, we will express this as:
x classes = 5 1/4 buses
Solving both expression
1 class = 3/4 buses
x classes = 5 1/4 buses
you will need to cross multiply from here:
3/4 * x = 1 * 5 1/4
3/4 x = 21/4
multiply both sides by 4
3x/4 * 4 = 21/4 * 4
3x = 21
Divide both sides by 3:
3x/3 = 21/3
x = 7
Therefore 7 classes are riding on the bus.
