1) find the area of the box:
1.5 * 1.2 * 1.4 = 2.52
2) How much will need to be filled with pebbles?
2.52 * 0.25 = 0.63
So only 0.63 ft^3 needs to be filled. If Jacob can only buy pebbles by cubic feet, then he will spend 2 dollars. If not, then he can spend $1.26
Hope this helps :)